run-command: restrict PATH search to executable files

In some situations run-command will incorrectly try (and fail) to
execute a directory instead of an executable file.  This was observed by
having a directory called "ssh" in $PATH before the real ssh and trying
to use ssh protoccol, reslting in the following:

	$ git ls-remote ssh://url
	fatal: cannot exec 'ssh': Permission denied

It ends up being worse and run-command will even try to execute a
non-executable file if it preceeds the executable version of a file on
the PATH.  For example, if PATH=~/bin1:~/bin2:~/bin3 and there exists a
directory 'git-hello' in 'bin1', a non-executable file 'git-hello' in
bin2 and an executable file 'git-hello' (which prints "Hello World!") in
bin3 the following will occur:

	$ git hello
	fatal: cannot exec 'git-hello': Permission denied

This is due to only checking 'access()' when locating an executable in
PATH, which doesn't distinguish between files and directories.  Instead
use 'is_executable()' which check that the path is to a regular,
executable file.  Now run-command won't try to execute the directory or
non-executable file 'git-hello':

	$ git hello
	Hello World!

which matches what execvp(3) would have done when asked to execute
git-hello with such a $PATH.

Reported-by: Brian Hatfield <bhatfield@google.com>
Signed-off-by: Brandon Williams <bmwill@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -159,6 +159,23 @@ int is_executable(const char *name)
 	return st.st_mode & S_IXUSR;
 }
 
+/*
+ * Search $PATH for a command.  This emulates the path search that
+ * execvp would perform, without actually executing the command so it
+ * can be used before fork() to prepare to run a command using
+ * execve() or after execvp() to diagnose why it failed.
+ *
+ * The caller should ensure that file contains no directory
+ * separators.
+ *
+ * Returns the path to the command, as found in $PATH or NULL if the
+ * command could not be found.  The caller inherits ownership of the memory
+ * used to store the resultant path.
+ *
+ * This should not be used on Windows, where the $PATH search rules
+ * are more complicated (e.g., a search for "foo" should find
+ * "foo.exe").
+ */
 static char *locate_in_PATH(const char *file)
 {
 	const char *p = getenv("PATH");
@@ -179,7 +196,7 @@ static char *locate_in_PATH(const char *file)
 		}
 		strbuf_addstr(&buf, file);
 
-		if (!access(buf.buf, F_OK))
+		if (is_executable(buf.buf))
 			return strbuf_detach(&buf, NULL);
 
 		if (!*end)

t/t0061-run-command.sh

@@ -37,6 +37,36 @@ test_expect_success !MINGW 'run_command can run a script without a #! line' '
 	test_cmp empty err
 '
 
+test_expect_success 'run_command does not try to execute a directory' '
+	test_when_finished "rm -rf bin1 bin2" &&
+	mkdir -p bin1/greet bin2 &&
+	write_script bin2/greet <<-\EOF &&
+	cat bin2/greet
+	EOF
+
+	PATH=$PWD/bin1:$PWD/bin2:$PATH \
+		test-run-command run-command greet >actual 2>err &&
+	test_cmp bin2/greet actual &&
+	test_cmp empty err
+'
+
+test_expect_success POSIXPERM 'run_command passes over non-executable file' '
+	test_when_finished "rm -rf bin1 bin2" &&
+	mkdir -p bin1 bin2 &&
+	write_script bin1/greet <<-\EOF &&
+	cat bin1/greet
+	EOF
+	chmod -x bin1/greet &&
+	write_script bin2/greet <<-\EOF &&
+	cat bin2/greet
+	EOF
+
+	PATH=$PWD/bin1:$PWD/bin2:$PATH \
+		test-run-command run-command greet >actual 2>err &&
+	test_cmp bin2/greet actual &&
+	test_cmp empty err
+'
+
 test_expect_success POSIXPERM 'run_command reports EACCES' '
 	cat hello-script >hello.sh &&
 	chmod -x hello.sh &&