mingw: unset PERL5LIB by default

Git for Windows ships with its own Perl interpreter, and insists on
using it, so it will most likely wreak havoc if PERL5LIB is set before
launching Git.

Let's just unset that environment variables when spawning processes.

To make this feature extensible (and overrideable), there is a new
config setting `core.unsetenvvars` that allows specifying a
comma-separated list of names to unset before spawning processes.

Reported by Gabriel Fuhrmann.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -921,6 +921,12 @@ relatively high IO latencies.  When enabled, Git will do the
 index comparison to the filesystem data in parallel, allowing
 overlapping IO's.  Defaults to true.
 
+core.unsetenvvars::
+	Windows-only: comma-separated list of environment variables'
+	names that need to be unset before spawning any other process.
+	Defaults to `PERL5LIB` to account for the fact that Git for
+	Windows insists on using its own Perl interpreter.
+
 core.createObject::
 	You can set this to 'link', in which case a hardlink followed by
 	a delete of the source are used to make sure that object creation

compat/mingw.c

@@ -212,6 +212,7 @@ enum hide_dotfiles_type {
 };
 
 static enum hide_dotfiles_type hide_dotfiles = HIDE_DOTFILES_DOTGITONLY;
+static char *unset_environment_variables;
 
 int mingw_core_config(const char *var, const char *value, void *cb)
 {
@@ -223,6 +224,12 @@ int mingw_core_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.unsetenvvars")) {
+		free(unset_environment_variables);
+		unset_environment_variables = xstrdup(value);
+		return 0;
+	}
+
 	return 0;
 }
 
@@ -1180,6 +1187,27 @@ static wchar_t *make_environment_block(char **deltaenv)
 	return wenvblk;
 }
 
+static void do_unset_environment_variables(void)
+{
+	static int done;
+	char *p = unset_environment_variables;
+
+	if (done || !p)
+		return;
+	done = 1;
+
+	for (;;) {
+		char *comma = strchr(p, ',');
+
+		if (comma)
+			*comma = '\0';
+		unsetenv(p);
+		if (!comma)
+			break;
+		p = comma + 1;
+	}
+}
+
 struct pinfo_t {
 	struct pinfo_t *next;
 	pid_t pid;
@@ -1198,9 +1226,12 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 	wchar_t wcmd[MAX_PATH], wdir[MAX_PATH], *wargs, *wenvblk = NULL;
 	unsigned flags = CREATE_UNICODE_ENVIRONMENT;
 	BOOL ret;
+	HANDLE cons;
+
+	do_unset_environment_variables();
 
 	/* Determine whether or not we are associated to a console */
-	HANDLE cons = CreateFile("CONOUT$", GENERIC_WRITE,
+	cons = CreateFile("CONOUT$", GENERIC_WRITE,
 			FILE_SHARE_WRITE, NULL, OPEN_EXISTING,
 			FILE_ATTRIBUTE_NORMAL, NULL);
 	if (cons == INVALID_HANDLE_VALUE) {
@@ -2437,6 +2468,8 @@ void mingw_startup(void)
 	/* fix Windows specific environment settings */
 	setup_windows_environment();
 
+	unset_environment_variables = xstrdup("PERL5LIB");
+
 	/* initialize critical section for waitpid pinfo_t list */
 	InitializeCriticalSection(&pinfo_cs);
 

t/t0029-core-unsetenvvars.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+
+test_description='test the Windows-only core.unsetenvvars setting'
+
+. ./test-lib.sh
+
+if ! test_have_prereq MINGW
+then
+	skip_all='skipping Windows-specific tests'
+	test_done
+fi
+
+test_expect_success 'setup' '
+	mkdir -p "$TRASH_DIRECTORY/.git/hooks" &&
+	write_script "$TRASH_DIRECTORY/.git/hooks/pre-commit" <<-\EOF
+	echo $HOBBES >&2
+	EOF
+'
+
+test_expect_success 'core.unsetenvvars works' '
+	HOBBES=Calvin &&
+	export HOBBES &&
+	git commit --allow-empty -m with 2>err &&
+	grep Calvin err &&
+	git -c core.unsetenvvars=FINDUS,HOBBES,CALVIN \
+		commit --allow-empty -m without 2>err &&
+	! grep Calvin err
+'
+
+test_done