https://lists.samba.org/archive/ccache/2014q4/001246.html

From 5d0f507a4162ac89e05ca633dbc8056454b798f5 Mon Sep 17 00:00:00 2001
From: Mike Frysinger <vapier@gentoo.org>
Date: Mon, 15 Sep 2014 18:15:02 -0400
Subject: [PATCH] do not rely on pids being unique

Linux supports creating pid namespaces cheaply and running processes
inside of them.  When you try to share a single cache among multiple
such runs, the fact that the code relies on pid numbers as globally
unique values quickly fails.  Instead, switch to standard mkstemp to
generate temp files for us.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
---
 ccache.c   | 12 ++++++------
 ccache.h   |  2 +-
 manifest.c |  2 +-
 stats.c    | 10 +++++++++-
 util.c     | 13 ++++++++-----
 5 files changed, 25 insertions(+), 14 deletions(-)

diff --git a/ccache.c b/ccache.c
index 02dbdfa..1dc0a06 100644
--- a/ccache.c
+++ b/ccache.c
@@ -526,8 +526,11 @@ to_cache(struct args *args)
 	unsigned added_files = 0;
 
 	tmp_stdout = format("%s.tmp.stdout.%s", cached_obj, tmp_string());
+	create_empty_file(tmp_stdout);
 	tmp_stderr = format("%s.tmp.stderr.%s", cached_obj, tmp_string());
+	create_empty_file(tmp_stderr);
 	tmp_obj = format("%s.tmp.%s", cached_obj, tmp_string());
+	create_empty_file(tmp_obj);
 
 	args_add(args, "-o");
 	args_add(args, tmp_obj);
@@ -579,7 +582,7 @@ to_cache(struct args *args)
 		int fd_result;
 		char *tmp_stderr2;
 
-		tmp_stderr2 = format("%s.tmp.stderr2.%s", cached_obj, tmp_string());
+		tmp_stderr2 = format("%s.2", tmp_stderr);
 		if (x_rename(tmp_stderr, tmp_stderr2)) {
 			cc_log("Failed to rename %s to %s: %s", tmp_stderr, tmp_stderr2,
 			       strerror(errno));
@@ -808,6 +808,7 @@ get_object_name_from_cpp(struct args *args, struct mdfour *hash)
 	}
 
 	path_stderr = format("%s/tmp.cpp_stderr.%s", temp_dir, tmp_string());
+	create_empty_file(path_stderr);
 	add_pending_tmp_file(path_stderr);
 
 	time_of_compilation = time(NULL);
@@ -815,6 +816,7 @@ get_object_name_from_cpp(struct args *args, struct mdfour *hash)
 	if (!direct_i_file) {
 		path_stdout = format("%s/%s.tmp.%s.%s",
 		                     temp_dir, input_base, tmp_string(), i_extension);
+		create_empty_file(path_stdout);
 		add_pending_tmp_file(path_stdout);
 
 		/* run cpp on the input file to obtain the .i */
@@ -838,11 +843,6 @@ get_object_name_from_cpp(struct args *args, struct mdfour *hash)
 		   can skip the cpp stage and directly form the
 		   correct i_tmpfile */
 		path_stdout = input_file;
-		if (create_empty_file(path_stderr) != 0) {
-			cc_log("Failed to create %s: %s", path_stderr, strerror(errno));
-			stats_update(STATS_ERROR);
-			failed();
-		}
 		status = 0;
 	}
 
diff --git a/ccache.h b/ccache.h
index 2bc7c87..43ef98d 100644
--- a/ccache.h
+++ b/ccache.h
@@ -130,7 +130,7 @@ size_t file_size(struct stat *st);
 int safe_open(const char *fname);
 char *x_realpath(const char *path);
 char *gnu_getcwd(void);
-int create_empty_file(const char *fname);
+int create_empty_file(char *fname);
 const char *get_home_directory(void);
 char *get_cwd();
 bool same_executable_name(const char *s1, const char *s2);
diff --git a/manifest.c b/manifest.c
index 7f02ede..47566d5 100644
--- a/manifest.c
+++ b/manifest.c
@@ -633,7 +633,7 @@ manifest_put(const char *manifest_path, struct file_hash *object_hash,
 	}
 
 	tmp_file = format("%s.tmp.%s", manifest_path, tmp_string());
-	fd2 = safe_open(tmp_file);
+	fd2 = mkstemp(tmp_file);
 	if (fd2 == -1) {
 		cc_log("Failed to open %s", tmp_file);
 		goto out;
diff --git a/stats.c b/stats.c
index 2111b65..4ed39c2 100644
--- a/stats.c
+++ b/stats.c
@@ -126,11 +126,18 @@ stats_write(const char *path, struct counters *counters)
 	size_t i;
 	char *tmp_file;
 	FILE *f;
+	int fd;
 
 	tmp_file = format("%s.tmp.%s", path, tmp_string());
-	f = fopen(tmp_file, "wb");
+	fd = mkstemp(tmp_file);
+	if (fd == -1) {
+		cc_log("Failed to open %s", tmp_file);
+		goto end;
+	}
+	f = fdopen(fd, "wb");
 	if (!f) {
 		cc_log("Failed to open %s", tmp_file);
+		close(fd);
 		goto end;
 	}
 	for (i = 0; i < counters->size; i++) {
@@ -138,6 +145,7 @@ stats_write(const char *path, struct counters *counters)
 			fatal("Failed to write to %s", tmp_file);
 		}
 	}
+	/* This also implicitly closes the fd. */
 	fclose(f);
 	x_rename(tmp_file, path);
 
diff --git a/util.c b/util.c
index 3b472de..cc630a6 100644
--- a/util.c
+++ b/util.c
@@ -195,7 +195,7 @@ copy_file(const char *src, const char *dest, int compress_dest)
 	struct stat st;
 	int errnum;
 
-	tmp_name = format("%s.%s.XXXXXX", dest, tmp_string());
+	tmp_name = format("%s.%s", dest, tmp_string());
 	cc_log("Copying %s to %s via %s (%s)",
 	       src, dest, tmp_name, compress_dest ? "compressed": "uncompressed");
 
@@ -427,7 +427,7 @@ tmp_string(void)
 	static char *ret;
 
 	if (!ret) {
-		ret = format("%s.%u", get_hostname(), (unsigned)getpid());
+		ret = format("%s.%u.XXXXXX", get_hostname(), (unsigned)getpid());
 	}
 
 	return ret;
@@ -884,12 +884,13 @@ gnu_getcwd(void)
 
 /* create an empty file */
 int
-create_empty_file(const char *fname)
+create_empty_file(char *fname)
 {
 	int fd;
 
-	fd = open(fname, O_WRONLY|O_CREAT|O_TRUNC|O_EXCL|O_BINARY, 0666);
+	fd = mkstemp(fname);
 	if (fd == -1) {
+		cc_log("Failed to create %s: %s", fname, strerror(errno));
 		return -1;
 	}
 	close(fd);
@@ -1134,7 +1135,9 @@ x_unlink(const char *path)
 		goto out;
 	}
 	if (unlink(tmp_name) == -1) {
-		result = -1;
+		/* If it was released in a race, that's OK. */
+		if (errno != ENOENT)
+			result = -1;
 	}
 out:
 	free(tmp_name);
-- 
2.1.2