Personally I like Python for what it is - a quick prototype language for writing simple utilities/scripts. Unfortunately when you start writing important infrastructure code using Python it quickly falls apart due to its dynamic typing. And all the breaking changes in Python3 didn’t help either.
A quick case study for go concurrency
A classical computer science problem is a worker queue - imagine you have N WorkItems to be divided between M workers (of course this implies N » M). This pattern has many application such as generating lots of data and insert into database, processing CPU-intensive work, etc. This can be easily achieved with go routine and channels. I recently spend a bit of time working on a similar problem with go and I’d like to share the pattern that I came up with.
An overview of MySQL, as well as how to obtain source, build and run tests
MySQL is one of the most widely used OpenSource relational databases and is used by many companies such as Amazon, Facebook, Google, Alibaba, etc. In my current job we deploy MySQL widely within the company, we had our MySQL 5.6 own fork and moving towards MySQL 8.0 currently in a branch. We also have an “new” storage engine built on top of RocksDB, not surprisingly called MyRocks, which lives under storage/rocksdb folder in the MySQL 5.6 fork.
On a 10000-feet view, the architecture of MySQL server looks like this:
If you dive deeper, an execution engine itself could include following pieces:
One of the most amazing features in MySQL is to swap the underlying storage engine while keeping the same upper layers - this way you can have different in-memory and on-disk representation of databases for different workloads, while keeping the same SQL execution functionality so the client wouldn’t even know the underlying storage engine have changed. The default storage engine is InnoDB - a B+ tree based database storage engine, and the one that we have is MyRocks which is built on top of RocksDB, a LSM-tree based database storage engine. There is an API layer called handler that storage engine need to implement/override. You can go to Comparison of MySQL database engines to see a list of common storage engines in MySQL.
Of course, the statement that they wouldn’t know the storage engine has changed is not entirely accurate. There are specific configurations you might need to tune / config the storage engine to your needs, and different storage engine has different performance / behavior / features / capabilities, so it’s not completely transparent.
You can obtain source code from MySQL website, but most folks probably would prefer a github mirror:
git clone https://github.com/mysql/mysql-server
This contains the latest MySQL 8.0.16.
In a typical Ubuntu system, you need to install following dependencies:
sudo apt install libssl-dev libzstd-dev libncurses5-dev libreadline-dev bison pkg-config
All my instructions below are tested on a Azure Linux Ubuntu 18.04 VM and on a MacBook Pro 2018. They may vary slightly due to your configuration/distribution if you are on a unix/linux system. Getting it to work on Windows requires installing OpenSSL binaries and GNU Bison. If you are using latest Visual Studio 2019, you may also need to apply a fix to boost 1.69.0 for a outdated VC workaround (a workaround for a workaround, essentially). Fortunately in most cases MySQL is pretty good about telling you exactly what is missing and where to download them.
Now let’s create a debug directory to store all our build files, and start the debug build:
mkdir debug
cd debug
cmake .. -DWITH_DEBUG=1 -DDOWNLOAD_BOOST=1 -DWITH_BOOST=~/boost_1_69_0
make
WITH_DEBUG=1 requests a debug build, which makes debugger easierDOWNLOAD_BOOST=1 WITH_BOOST=~/boost_1_69_0 downloads the boost at ~/boost_1_69_0 (that’s the version MySQL is asking for), and will skip the downloading if it is already thereOne the build is done, you can find everything under debug/bin.
Don’t change the build directory after the fact once you done the build. The directory name is remembered and changing that naming requires a rebuild.
To validate that we indeed have a working MySQL build, let’s try running a quick test called select_all.
To run any test, there is a script mysql-test-run.pl located under the mysql-test directory from the build directory, and it takes a test name in the form of <testname> or <testsuite>.<testname>:
cd debug/mysql-test
./mysql-test-run.pl select_all
This runs the test under mysql-test/t/select_all.test with baseline mysql-test/r/select_all.result. It runs a simple test language containing test directives/commands and SQL commands, and compare the output with the baseline. If the output diverges from the baseline the test would fail, otherwise it would pass. Simple enough, right?
Actually, not quite. The testing of MySQL can get quite complicated when it involves multiple connections / servers communicating with each other. And stablizing the results so that they are not affected by external environment / code changes can be also an headache.
Here is what you should see:
[~/local/github/mysql-server/debug/mysql-test, 8.0, 51s, SUDO]: ./mysql-test-run.pl select_all
Logging: /home/yzha/local/github/mysql-server/mysql-test/mysql-test-run.pl select_all
MySQL Version 8.0.16
Checking supported features
- Binaries are debug compiled
Using 'all' suites
Collecting tests
Checking leftover processes
Removing old var directory
Creating var directory '/home/yzha/local/github/mysql-server/debug/mysql-test/var'
Installing system database
Using parallel: 1
==============================================================================
TEST NAME RESULT TIME (ms) COMMENT
------------------------------------------------------------------------------
[100%] main.select_all [ pass ] 36259
------------------------------------------------------------------------------
The servers were restarted 0 times
The servers were reinitialized 0 times
Spent 36.259 of 70 seconds executing testcases
Running a test seems straight-forward enough. If you want to launch mysql server and run some SQL commands against it, it takes a bit of work.
First we need to have mysqld initializes a blank data directory:
cd debug/bin
./mysqld --initialize
In Windows you’ll need to add
--consoleto write error output to screen. Otherwise it’s only available in the .err log file.
You should see:
2019-06-05T05:16:31.376510Z 0 [System] [MY-013169] [Server] /datadrive/github/mysql-server/debug/runtime_output_directory/mysqld (mysqld 8.0.16-debug) initializing of server in progress as process 70030
2019-06-05T05:16:44.066787Z 5 [Note] [MY-010454] [Server] A temporary password is generated for root@localhost: <.....>
2019-06-05T05:16:53.317610Z 0 [System] [MY-013170] [Server] /datadrive/github/mysql-server/debug/runtime_output_directory/mysqld (mysqld 8.0.16-debug) initializing of server has completed
Note the temporary password generated in the second line. You’ll need it later.
This means that mysqld has successfully initialized at debug/data directory:
'#innodb_temp' auto.cnf ca-key.pem ca.pem client-cert.pem client-key.pem ib_buffer_pool ib_logfile0 ib_logfile1 ibdata1 mysql mysql.ibd performance_schema private_key.pem public_key.pem server-cert.pem server-key.pem sys undo_001 undo_002
Now we can finally start the server:
cd debug/bin
./mysqld --debug
In Windows you’ll need to add
--consoleto write error output to screen. Otherwise it’s only available in the .err log file.
--debug switch means we start the mysql server in debug mode.
Now launch another terminal / TMUX window / whatever, and connect to the mysql server:
cd debug/bin
./mysql -uroot --socket=/tmp/mysql.sock -p
You should see:
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 28
Server version: 8.0.16-debug Source distribution
Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
However we are not quite done yet. Mysql will ask us to change the password - you can do it by using the following:
mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY "<newpassword>";
Now any future login can be done using this new password you just gave.
Finally we can run some SQL command!
mysql> SELECT @@version;
+--------------+
| @@version |
+--------------+
| 8.0.16-debug |
+--------------+
1 row in set (0.00 sec)
Before I leave, let me address a question that is absolutely going to be asked - how do I terminate the server gracefully? CTRL+C doesn’t work anymore.
The right way is to use mysqladmin:
cd debug/bin
./mysqladmin -uroot -p shutdown
You’ll see the server waving goodbye:
2019-06-05T05:54:09.028071Z 30 [System] [MY-013172] [Server] Received SHUTDOWN from user root. Shutting down mysqld (Version: 8.0.16-debug).
2019-06-05T05:54:10.670124Z 0 [System] [MY-010910] [Server] /datadrive/github/mysql-server/debug/runtime_output_directory/mysqld: Shutdown complete (mysqld 8.0.16-debug) Source distribution
In Linux/Mac debugging is relatively straightforward and isn’t any different from other applications. My personal recommendation is to use Visual Studio Code and setup lldb/gdb debugging there.
For Windows, the obvious choice is Visual Studio (2019 Community Edition is the one I’ve tested on). However it looks like mysqld is launching another mysqld instance that does the real work, so F5 debugging that mysqld from Visual Studio requires Child Process Debugging Power Tool in order for your breakpoints to hit since they need to be set in the child mysqld process, not the parent. Of course attaching to the correct mysqld would always work regardless without the help of the Child Process Debugging Power Tool.
I’m planning a series of articles that will go through many interesting aspects of MySQL:
I’m also planning to write about MyRocks, as well as RocksDB / LevelDB / InnoDB, but I’ll priorize MySQL articles first as they lay down a nice foundation for rest of the stuff and this also serves as documentation when people get lost in the vast amount of MySQL source code.
Let me know what do you think about the article and/or if you are running into issues. Feel free to suggest topics as well. But I probably can’t help much with your DBA questions…
How to get GDB to work on MacOSX Mojave
Starting from Mac OS X 10.5 (Leopard), Apple starts to lock down the system further and debuggers like GDB now have to be code signed. There is a great article describing steps to get it to work. However, there are a lot of conflicting information on the web and people are having trouble with some of those instructions, myself included. So I’d like to document what I did to get it to work, and highlight the issues I ran into:
On a high-level you need to perform these steps:
The article has detailed steps on these steps so I’m not going to repeat them.
A few gotchas that I ran into myself:
Starting program: /Users/yzha/github/mysql-server/debug/runtime_output_directory/mysqld
Unable to find Mach task port for process-id 55009: (os/kern) failure (0x5).
(please check gdb is codesigned - see taskgated(8))
Double check if you had the proper entitlements in a XML file and pass to codesign when you are signing GDB. Many articles on the web in fact didn’t have the entitlement step as it likely is a new requirement 10.14+.
During startup program terminated with signal ?, Unknown signal.
or this:
/Users/yizhang/github/leveldb/debug/db_test": not in executable format: file format not recognized
Make sure you stay off GDB 8.2! Upgrade to 8.3 (available in Homebrew already) or downgrade to 8.0.
Sorting structured data using memcmp-friendly encoding part 2 - sorting floats
In the last post we’ve discussed converting integers and strings into a memcmp / byte-comparable format for faster comparison (but at the trade off of doing decoding/encoding at reading/writing). In this post let’s take a look at how do we do the same for floating pointers.
Making cherry-pick work across file renames
Recently I need to port over some changes using cherry-pick and that usually works fine without any issues (except for occasional conflicts), but this time the actual file foo.cc was renamed to bar.cc. In such case git cherry-pick simply gives up and simply tells you the old file you are changing has been deleted. As far as I can tell there isn’t a good way to resolve the conflict.
There are a couple of ways to address this issue. But the easiest way I found is to just rename the file back to the original name where you had made the change on, in order to make git happy. Once that’s done, cherry-picking would work fine as usual. Now just rename the file back to the ‘new’ name. Squash the change.
This can be illustrated in following example - assuming:
# Create the target branch as usual
git checkout -b your-target-branch
# Rename bar.cc back to foo.cc to make git cherry-pick happy
git mv bar.cc foo.cc
git commit -m "Make git happy"
# Cherry-pick as usual
git cherry-pick -x <commit>
# Rename it back
git mv foo.cc bar.cc
git commit -m "Rename back"
# Squash the 3 commits into one
git rebase -i HEAD~3
In the rebase file, you’ll see:
pick 95be80db682 Make git happy
pick 3d74c6c9e13 Cherry-pick commit blah
pick 238e3c51354 Rename back
Change to:
pick 95be80db682 Make git happy
s 3d74c6c9e13 Cherry-pick commit blah
s 238e3c51354 Rename back
Here s means squash with previous commit.
Just remember in commit message deleting the first and third unrelated commit.
And now you are all set!
A few days ago I was looking into a deadlock issue that is caused by a behavioral difference between MySQL storage engine transaction behavior in repeatable reads. This leads me to dig deeper into repeatable read behavior in InnoDB and what I found is quite interesting:
Before we dig deeper, let’s revisit some of the basics of database isolation levels. You can refer to my earlier post for a more detailed explanation / comparison. Database isolation level defines the behavior of data read/write operations within transactions, and those can have a signficant impact to protecting the data integrity of your application. Repeatable reads guaratees that you would always observe the same value once you read it, and it would never change unless you’ve made the change yourself, giving you the illusion that it is exclusively owned by you and there is no one else. Of course, this isn’t true in practice as there are pessimistic locking and optimistic locking that defines the behavior when write conflict occurs.
The art of argument parsing and policy transparency
When working with MySQL the often most frustrating part is getting strange connection errors. I’ve wasted two hours trying to connect to a MySQL server using TCP port (unix domain sockets works fine) and I’ll talk about why it didn’t work, and as usual we’ll dive into the code to understand exactly why.
To simplify the problem, let’s say I have MySQL server at port 13010 and bound to localhost, with user name root and empty password (don’t do that in production):
[~/mysql]: mysql -p 13010 -h localhost -u root
Enter password:
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)
This is typical error many people will run into and you can find many similar posts that discuss the problem but few ever got to the bottom of it. Let’s jump right in.
Obviously when I write -p 13010 I meant to tell mysql client to connect to server using port 13010, but that’s not quite right:
[~/mysql]: mysql --help
-p, --password[=name]
Password to use when connecting to server. If password is
-P, --port=# Port number to use for connection or 0 for default
So I actually told mysql the password is 13010 instead. Supporting both -p and -P is a apparently very bad idea.
Linux tools often have excessive amount of short options, like this one from man page for ls:
ls [-ABCFGHLOPRSTUW@abcdefghiklmnopqrstuwx1] [file …]
Personally I think they should go easy and only include the most common ones rather than using the entire alphabet.
However, the mystery is not yet solved. Note that we have been asked to enter the password, which explains why most people never suspected -p actually means password. Put in other words - if -p means password, why is this command is still asking for password?
The answer lies in the source code:
for (optend= cur_arg; *optend; optend++)
{
opt_found= 0;
for (optp= longopts; optp->name; optp++)
{
if (optp->id && optp->id == (int) (uchar) *optend)
{
/* Option recognized. Find next what to do with it */
opt_found= 1;
if (optp->arg_type == REQUIRED_ARG ||
optp->arg_type == OPT_ARG)
{
if (*(optend + 1))
{
/* The rest of the option is option argument */
argument= optend + 1;
/* This is in effect a jump out of the outer loop */
optend= (char*) " ";
}
else
{
if (optp->arg_type == OPT_ARG)
{
if (optp->var_type == GET_BOOL)
*((my_bool*) optp->value)= (my_bool) 1;
if (get_one_option && get_one_option(optp->id, optp, argument))
return EXIT_UNSPECIFIED_ERROR;
continue;
}
/* Check if there are more arguments after this one */
argument= *++pos;
(*argc)--;
The *(optend + 1) is the most interesting part. If a short-form option is being recognized, the rest immediately following the short option is treated as argument:
if (*(optend + 1))
{
/* The rest of the option is option argument */
argument= optend + 1;
/* This is in effect a jump out of the outer loop */
optend= (char*) " ";
Given that we are not passing -p13010, the 13010 part is ignored.
But wait, why does -h localhost work fine?
Just keep looking:
if (optp->arg_type == OPT_ARG)
{
if (optp->var_type == GET_BOOL)
*((my_bool*) optp->value)= (my_bool) 1;
if (get_one_option && get_one_option(optp->id, optp, argument))
return EXIT_UNSPECIFIED_ERROR;
continue;
}
/* Check if there are more arguments after this one */
if (!pos[1])
{
return EXIT_ARGUMENT_REQUIRED;
}
argument= *++pos;
(*argc)--;
So if the argument is an optional arg, it’ll give up and only check for immediate following argument. Otherwise, for OPT_REQUIRED, it assumes the next one is the argument.
Let’s take a look at where they are defined:
{"password", 'p',
"Password to use when connecting to server. If password is not given it's asked from the tty.",
0, 0, 0, GET_PASSWORD, OPT_ARG, 0, 0, 0, 0, 0, 0},
{"host", 'h', "Connect to host.", ¤t_host,
¤t_host, 0, GET_STR_ALLOC, REQUIRED_ARG, 0, 0, 0, 0, 0, 0},
As expected, password is optional and host is required.
Also, note that how it never checked for ‘=’? So the syntax -p=abc wouldn’t work as expected as well. And hilariously =abc would become the password. For arguments with a bit more error checking like port, the error message is a bit better:
[~/mysql]: mysql -P=13010
mysql: [ERROR] Unknown suffix '=' used for variable 'port' (value '=13010')
mysql: [ERROR] mysql: Error while setting value '=13010' to 'port'
Note the ‘=13010’ part?
OK. Let’s try again:
[~/mysql/mysql-fork]: mysql -P 13010 -h localhost -u root
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)
Still doesn’t work. We know it’s not the parsing of -P because port is OPT_REQUIRED:
{"port", 'P', "Port number to use for connection or 0 for default to, in "
"order of preference, my.cnf, $MYSQL_TCP_PORT, "
#if MYSQL_PORT_DEFAULT == 0
"/etc/services, "
#endif
"built-in default (" STRINGIFY_ARG(MYSQL_PORT) ").",
&opt_mysql_port,
&opt_mysql_port, 0, GET_UINT, REQUIRED_ARG, 0, 0, 0, 0, 0, 0},
Note the error message socket '/var/lib/mysql/mysql.sock. This is for domain socket.
To confirm this is the issue, let’s search for the actual error message:
const char *client_errors[]=
{
"Unknown MySQL error",
"Can't create UNIX socket (%d)",
"Can't connect to local MySQL server through socket '%-.100s' (%d)",
The client_errors are looked up from error codes:
#define ER(X) (((X) >= CR_ERROR_FIRST && (X) <= CR_ERROR_LAST)? \
client_errors[(X)-CR_ERROR_FIRST]: client_errors[CR_UNKNOWN_ERROR])
And the 3rd error is CR_SOCKET_CREATE_ERROR:
#define CR_ERROR_FIRST 2000 /*Copy first error nr.*/
#define CR_UNKNOWN_ERROR 2000
#define CR_SOCKET_CREATE_ERROR 2001
Searching for that leads us back to client.cc:
if (!net->vio &&
(!mysql->options.protocol ||
mysql->options.protocol == MYSQL_PROTOCOL_SOCKET) &&
(unix_socket || mysql_unix_port) &&
(!host || !strcmp(host,LOCAL_HOST)))
{
my_socket sock= socket(AF_UNIX, SOCK_STREAM, 0);
DBUG_PRINT("info", ("Using socket"));
if (sock == SOCKET_ERROR)
{
set_mysql_extended_error(mysql, CR_SOCKET_CREATE_ERROR,
unknown_sqlstate,
ER(CR_SOCKET_CREATE_ERROR),
socket_errno);
DBUG_RETURN(STATE_MACHINE_FAILED);
}
So this means by default we are connecting using Unix domain socket, and only if host is not specifed or is localhost!
Programs should be transparent about its policies, and give information about what it is doing. If that can end up being too verbose, add a verbose option. I’ll write a separate post about this because I’ve been bitten too many times by similar issues and now my favorite past-time is to add print/printf.
So there are two ways to fix this:
127.0.0.1. This fails the UNIX socket check and will fallback to TCP.--protocol tcp to force using TCP.So the right command would be:
mysql -P 13010 -h localhost -u root --protocol tcp
or
mysql -P 13010 -h 127.0.0.1 -u root
These two problems can be easily avoided by adding more messages to the mysql client, such as:
Trying to connect to UNIX domain socket localhost...
Connecting to database `12310`.
These would’ve avoided wasting collectively god knows how much time wasted. Maybe I should submit a patch when I get a chance.
The gotchas:
mysql short-option with optional args only accept arguments when they immediately follow the option, such as ‘-pmypassword’. Specifying as ‘-p blah’ and blah will be interpreted as current database. Short option with required args don’t have this problem.
When there is no protocol specified, mysql will try to connect as UNIX domain socket if connecting to localhost or host isn’t specified. To work around it, use IP address instead of localhost, or specify protocol explicitly using --protocol.
Going linux full time
In my new job, no one cares about windows.
Every single developer (with a few exceptions) use MacBook Pro, and connect to their linux VM to get work done. Some people have the trash can MacPro. You get the idea. Being in Microsoft for ~12 years, this is admittingly a interesting adventure. Even though in several Microsoft projects in the past that I have been working on had linux versions (CoreCLR, Service Fabric, etc), most development is still done in Windows, and then ported to Linux/Mac. Whenever occasionally you wonder into the no-man’s land in Linux where the project tooling / infrastructure is falling significantly behind, you want to pull your hair out. Not Linux’s fault - but a matter of priority. In some extreme cases you’d wonder how even one can put out a linux version out at all.
Not anymore. Now linux (or Mac, if you count that in) is the full time job.
After a few weeks of research and practice, I’ve been happyily chugging along with TMUX + VIM + MOSH with my custom key bindings. In this article I’ll talk about a bit of my experience of making the transition.
Let’s get this one out of the way first. There is no replacement for Visual Studio. Period. The code completion (or Intelli-Sense) and debugging is simply unmatched by anything else in the market. VS Code is awesome in terms of just browsing code and doing some occasional debugging, but for writing code it is just OK as the “inteli-sense” (forgive my Microsoft VS Jargon) can be a hit or miss. Vim is good for text editing, and with plugins you can get some basic stuff to work, but again it’s no where near the quality of experience of Visual Studio. Usually it’s a love/hate relationship with Visual Studio - it’s kinda slow and some times buggy, but you can’t live without it. Well, you can, but you don’t want to.
Nowadays I use vim or VS Code / Atom for writing code, and gdb for debugging.
Being an reasonably experienced WinDbg user, Gdb’s command line taking a bit getting used to, but that’s about it. GDB also supports a TUI mode that shows the integrated text window for source/register/etc and a command window. It’s not great as many simple key bindings stop working in that mode (taken over by the TUI component) but as long as I can see a source code “window” over SSH I’m happy.
TMUX is a terminal multiplexer. With TMUX you won’t lose your working state - even if you disconnect from SSH, just ‘tmux attach’ you’ll resume where you left off. In this sense it is equivalent to a Windows Remote Desktop session.
The most powerful part is that it also allow you to break the terminal into multiple panes and windows, and this way you don’t have to leave the terminal and can easily switch between many different tasks with quick shortcuts. No more need to manage windows - everything is within the terminal. It’s like a virtual desktop for terminals. It’s build in the way that you barely had to touch the mouse anymore. Well, until you move to the browser, that is.
In my Microsoft job I use vim for simple editing purposes, and I like the vim way of thinking so much that I put all my editors into vim mode / vim plugin / vim key bindings. These days I found myself spending even more time in vim over SSH and so I invested more time finding better VIM configurations and plugins.
I use junegunn/vim-plug as my VIM plugin manager. It’s pretty minimal and gets the job done.
This is the list of plugins I use:
In my old job I usually “remote” into my development machines at office - and “remote” means “Windows Remote Desktop”. In a reasonable connection it is actually quite nice - there is little lag and you almost feel you are working on a local machine, with all the graphical UI - it’s really amazing.
With linux, you fallback to the good old text-based SSH. It’s kinda amazing in its own way that you can have text-based remote protocol for complicated full screen programs like vim. You don’t get graphical UI this way - but for the most part you don’t need to, and it’s usually blazing fast.
Mosh improves over SSH that it is async (doesn’t wait for server response) so it feels even more responsive. The trade-off is that it can get a bit jarring when you type something and it does’t react correctly initially.
Windows Commmand Prompt is fine. It works. I still remember I learned my first DOS commands at a 33MHZ 386DX. But it hadn’t changed much since then. ConEmu is a popular terminal and some people (especally admins) use PowerShell as well. But none of those match the flexiblity of linux shells - they just have so much more to offer. You can switch between different shells, adding customizations, even plugins.
For now I’m using ZSH with oh-my-zsh. It has fantastic themes and plugins. My favorite features are:
With all these, and throw in a few custom key bindings, the plain shell / windows command prompt just seems so boring.
However, tweaking these tools so that they work for you takes time. I find myself spending quite a bit of time tweaking the configurations to make it work better for me - and the time spent paid off. All the different configuration options are indeed quite overwhelming if starting from scratch so I use Awesome dotfiles project as my starting point for tweaking and forked my own version yizhang82/dotfiles. There are a lot of things that I like about the way the things are setup:
~/dotfiles - this way things can be automatically synchronized through a git pull. This is actually quite brilliant.Of course, many of the configurations there are already pretty good and is perfect as a starting point for my own configurations.
It contains all my TMUX, ZSH, VIM configurations, and by simplying cloning and running a script it goes into a new machine effortlessly. Most of these is done by the original author and I’m simply tweaking it to my needs.
It did take a bit getting used to, but I’m happy to report that I now feel very much productive roughly on the same level of productivity when I’m working on Windows (if not more). I do miss having a fully integrated Visual Studio experience, but the command line experience (with TMUX, etc) in Linux is so much better that it more than makes up for that. Of course, at the end of the day, what matters is getting the job done - just use the right tool for the job. In a future post I can get into a bit more details with my experience with these tools and share some of my learnings/tips.
P.S. I still use Windows at home. I have custom built (by myself) PC that has i7 4770K, 32G RAM, nVidia 2080 RTX mostly for gaming. I think Windows has mostly lost the mindshare of developers these days, but it’s still the OS for gamers, and will be for quite some time.
Objects with transactional semantics need move support
OK. I must admit this probably isn’t the best title out there.
Let’s imagine I have an object that represents a file in a transaction. Recall that transaction needs to be all-or-nothing - if the transaction is complete the files can be kept around / moved to the final destination, otherwise they need to be deleted.
The more or less obvious idea that comes to mind is to represent this with a TxnFile class (TransactionalFile). This is the best part I love about C++, BTW - very clean scoped / destruction semantics.
class TxnFile {
public:
TxnFile(const std::string &file)
: m_file(file), m_committed(false) {
}
~TxnFile() {
if (!m_committed) {
std::remove(file);
}
}
const std::string &get_file() { return m_file; }
void commit() {
m_comitted = true;
}
}
private:
std::string m_file;
bool m_committed;
};
OK. So far so good. Let’s actually implement that business logic:
std::vector<TxnFile> txn_files;
// Collect the files
for (auto &file : some_files) {
txn_files.emplace_back(file);
}
// Do something with them. If exception is thrown we'll remove the files
for (auto txn_file : txn_files) {
do_some_work(txn_file.get_file());
}
// If all is well, commit
for (auto txn_file : txn_files) {
txn_file.commit();
}
Looks rather straight-forward, right? If you try this out yourself, you’ll soon realize something is off - the files are being deleted for no reason at all!
The problem itself is obvious-ish: it really should’ve been a auto & as otherwise are constructing copies of TxnFile and upon destruction will remove the file!
// If all is well, commit
for (auto &txn_file : txn_files) {
do_some_work(txn_file.get_file());
}
However, we are not done yet. The problem is still happening - and in some cases, the files are even removed before we actually do work!
The problem, perhaps not that surprisingly, lies with the std::vector class. When expanding size of std::vector, STL will try to create a new block of memory, and copy/move the memory to it. If the class doesn’t have a move constructor, it’ll default to copy, and destroy the old one - which isn’t unlike the auto txn_file problem we discussed earlier, though a bit more subtle to catch.
Let’s try fixing it:
class TxnFile {
public:
TxnFile(const std::string &file)
: m_file(file), m_committed(false) {
}
TxnFile(const TxnFile &&file) {
m_file = std::move(file.m_file);
m_committed = file.committed;
}
~TxnFile() {
if (!m_committed) {
std::remove(file);
}
}
const std::string &get_file() { return m_file; }
void commit() {
m_comitted = true;
}
}
private:
std::string m_file;
bool m_committed;
};
Looks reasonable, right? Actually the problem is still there! The problem is that you now have a const r-value reference const TxnFile &&. This means that even though you have a r-value reference, you can’t change it at all - and what’s the point of that if you want the move semantics? The right way is to use a regular r-value reference TxnFile &&. Keep in mind declaring move constructor disable the copy constructor so you shouldn’t run into this problem again. But just for better clarifying the intention, it’s a good practice to delete the copy constructor explicitly.
class TxnFile {
public:
TxnFile(const std::string &file)
: m_file(file), m_committed(false) {
}
TxnFile(const TxnFile &) = delete;
TxnFile &operator =(const TxnFile &) = delete;
TxnFile(TxnFile &&rhs)
: m_file(std::move(rhs.m_file), m_committed(rhs.m_committed) {
}
TxnFile &operator = (TxnFile &&rhs) {
reset();
m_file = std::move(that.m_file);
m_committed = that.m_committed;
}
~TxnFile() {
reset();
}
void reset() {
if (!m_committed) {
std::remove(file);
}
}
const std::string &get_file() { return m_file; }
void commit() {
m_comitted = true;
}
}
private:
std::string m_file;
bool m_committed;
};
OK. We are finally done. Personally I think C++ has grown to the point that it is too complicated for most people and the interaction between different features can lead to really surprising behaviors. Even if you want to write a simple class there is already too much things to consider (copy/move semantics). And don’t get me started on template meta programming. However, if you stick to a relatively sane subset of C++, maybe you’ll be fine. Just maybe. I’ve been working in large C++ codebases profesionally for 14+ years and I still make stupid mistakes.
