Improve error messages when temporary file creation fails

Before, when creating a temporary file failed, a generic 'Unable to create
temporary file' message was printed. In some cases this could lead to
confusion as to which directory should be checked for correct permissions etc.

This patch adds the template for the temporary filename to the error message,
converting it to an absolute path if needed. A test verifies that the template
is indeed printed when pointing to a nonexistent or unwritable directory.

A copy of the original template is made in case mkstemp clears the template.

Signed-off-by: Arnout Engelen <arnouten@bzzt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -434,6 +434,7 @@ TEST_PROGRAMS_NEED_X += test-string-pool
 TEST_PROGRAMS_NEED_X += test-svn-fe
 TEST_PROGRAMS_NEED_X += test-treap
 TEST_PROGRAMS_NEED_X += test-index-version
+TEST_PROGRAMS_NEED_X += test-mktemp
 
 TEST_PROGRAMS = $(patsubst %,%$X,$(TEST_PROGRAMS_NEED_X))
 

t/t0070-fundamental.sh

@@ -12,4 +12,17 @@ test_expect_success 'character classes (isspace, isalpha etc.)' '
 	test-ctype
 '
 
+test_expect_success 'mktemp to nonexistent directory prints filename' '
+	test_must_fail test-mktemp doesnotexist/testXXXXXX 2>err &&
+	grep "doesnotexist/test" err
+'
+
+test_expect_success POSIXPERM 'mktemp to unwritable directory prints filename' '
+	mkdir cannotwrite &&
+	chmod -w cannotwrite &&
+	test_when_finished "chmod +w cannotwrite" &&
+	test_must_fail test-mktemp cannotwrite/testXXXXXX 2>err &&
+	grep "cannotwrite/test" err
+'
+
 test_done

test-mktemp.c

@@ -0,0 +1,14 @@
+/*
+ * test-mktemp.c: code to exercise the creation of temporary files
+ */
+#include "git-compat-util.h"
+
+int main(int argc, char *argv[])
+{
+	if (argc != 2)
+		usage("Expected 1 parameter defining the temporary file template");
+
+	xmkstemp(xstrdup(argv[1]));
+
+	return 0;
+}

wrapper.c

@@ -198,10 +198,22 @@ FILE *xfdopen(int fd, const char *mode)
 int xmkstemp(char *template)
 {
 	int fd;
+	char origtemplate[PATH_MAX];
+	strlcpy(origtemplate, template, sizeof(origtemplate));
 
 	fd = mkstemp(template);
-	if (fd < 0)
-		die_errno("Unable to create temporary file");
+	if (fd < 0) {
+		int saved_errno = errno;
+		const char *nonrelative_template;
+
+		if (!template[0])
+			template = origtemplate;
+
+		nonrelative_template = make_nonrelative_path(template);
+		errno = saved_errno;
+		die_errno("Unable to create temporary file '%s'",
+			nonrelative_template);
+	}
 	return fd;
 }
 
@@ -321,10 +333,22 @@ int gitmkstemps(char *pattern, int suffix_len)
 int xmkstemp_mode(char *template, int mode)
 {
 	int fd;
+	char origtemplate[PATH_MAX];
+	strlcpy(origtemplate, template, sizeof(origtemplate));
 
 	fd = git_mkstemp_mode(template, mode);
-	if (fd < 0)
-		die_errno("Unable to create temporary file");
+	if (fd < 0) {
+		int saved_errno = errno;
+		const char *nonrelative_template;
+
+		if (!template[0])
+			template = origtemplate;
+
+		nonrelative_template = make_nonrelative_path(template);
+		errno = saved_errno;
+		die_errno("Unable to create temporary file '%s'",
+			nonrelative_template);
+	}
 	return fd;
 }