[PATCH] help: correct behavior for is_executable on Windows

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From: Heiko Voigt <hvoigt@xxxxxxxxxx>

The previous implementation said that the filesystem information on
Windows is not reliable to determine whether a file is executable. To
gather this information it was peeking into the first two bytes of a
file to see whether it looks executable.

Apart from the fact that on Windows executables are defined as such by
their extension (and Git has special code to support "executing" scripts
when they have a she-bang line) it leads to serious performance
problems: not only do we have to open many files now, it gets even
slower when a virus scanner is running.

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.

Signed-off-by: Heiko Voigt <heiko.voigt@xxxxxxx>
Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx>
---
Published-As: https://github.com/dscho/git/releases/tag/help-is-exe-v1
Fetch-It-Via: git fetch https://github.com/dscho/git help-is-exe-v1

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

diff --git a/help.c b/help.c
index 53e2a67e00..bc6cd19cf3 100644
--- a/help.c
+++ b/help.c
@@ -105,7 +105,22 @@ static int is_executable(const char *name)
 		return 0;
 
 #if defined(GIT_WINDOWS_NATIVE)
-{	/* cannot trust the executable bit, peek into the file instead */
+	/*
+	 * On Windows there is no executable bit. The file extension
+	 * indicates whether it can be run as an executable, and Git
+	 * has special-handling to detect scripts and launch them
+	 * through the indicated script interpreter. We test for the
+	 * file extension first because virus scanners may make
+	 * it quite expensive to open many files.
+	 */
+	if (ends_with(name, ".exe"))
+		return S_IXUSR;
+
+{
+	/*
+	 * Now that we know it does not have an executable extension,
+	 * peek into the file instead.
+	 */
 	char buf[3] = { 0 };
 	int n;
 	int fd = open(name, O_RDONLY);
@@ -113,8 +128,8 @@ static int is_executable(const char *name)
 	if (fd >= 0) {
 		n = read(fd, buf, 2);
 		if (n == 2)
-			/* DOS executables start with "MZ" */
-			if (!strcmp(buf, "#!") || !strcmp(buf, "MZ"))
+			/* look for a she-bang */
+			if (!strcmp(buf, "#!"))
 				st.st_mode |= S_IXUSR;
 		close(fd);
 	}

base-commit: 4e59582ff70d299f5a88449891e78d15b4b3fabe
-- 
2.11.1.windows.prerelease.2.9.g3014b57



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