[PATCH v2] help: improve is_executable() on Windows

From: Heiko Voigt <hvoigt@hvoigt.net>

On Windows, executables need to have the file extension `.exe`, or they are not executables. Hence, to support scripts, Git for Windows also looks for a she-bang line by opening the file in question, and executing it via the specified script interpreter.

To figure out whether files in the `PATH` are executable, `git help` has code that imitates this behavior. With one exception: it *always* opens the files and looks for a she-bang line *or* an `MZ` tell-tale (nevermind that files with the magic `MZ` but without file extension `.exe` would still not be executable).

Opening this many files leads to performance problems that are even more serious when a virus scanner is running. Therefore, let's change the code to look for the file extension `.exe` early, and avoid opening the file altogether if we already know that it is executable.

See the following measurements (in seconds) as an example, where we execute a simple program that simply lists the directory contents and calls open() on every listed file:

With virus scanner running (coldcache):

$ ./a.exe /libexec/git-core/ before open (git-add.exe): 0.000000 after open (git-add.exe): 0.412873 before open (git-annotate.exe): 0.000175 after open (git-annotate.exe): 0.397925 before open (git-apply.exe): 0.000243 after open (git-apply.exe): 0.399996 before open (git-archive.exe): 0.000147 after open (git-archive.exe): 0.397783 before open (git-bisect--helper.exe): 0.000160 after open (git-bisect--helper.exe): 0.397700 before open (git-blame.exe): 0.000160 after open (git-blame.exe): 0.399136 ...

With virus scanner running (hotcache):

$ ./a.exe /libexec/git-core/ before open (git-add.exe): 0.000000 after open (git-add.exe): 0.000325 before open (git-annotate.exe): 0.000229 after open (git-annotate.exe): 0.000177 before open (git-apply.exe): 0.000167 after open (git-apply.exe): 0.000150 before open (git-archive.exe): 0.000154 after open (git-archive.exe): 0.000156 before open (git-bisect--helper.exe): 0.000132 after open (git-bisect--helper.exe): 0.000180 before open (git-blame.exe): 0.000718 after open (git-blame.exe): 0.000724 ...

With this patch I get:

$ time git help git Launching default browser to display HTML ...

real 0m8.723s user 0m0.000s sys 0m0.000s

and without

$ time git help git Launching default browser to display HTML ...

real 1m37.734s user 0m0.000s sys 0m0.031s

both tests with cold cache and giving the machine some time to settle down after restart.

[jes: adjusted the commit message]

Signed-off-by: Heiko Voigt <heiko.voigt@mahr.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
Published-As: https://github.com/dscho/git/releases/tag/help-is-exe-v2
Fetch-It-Via: git fetch https://github.com/dscho/git help-is-exe-v2

 help.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)