Do not rely on pids being unique

Based on a patch by Mike Frysinger <vapier@gentoo.org>:

"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."

ccache.c

@@ -613,8 +613,11 @@ 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);
@@ -666,7 +669,7 @@ 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));
@@ -808,13 +811,21 @@ 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);
 
 	if (!direct_i_file) {
-		path_stdout = format("%s/%s.tmp.%s.%s",
-		                     temp_dir, input_base, tmp_string(), i_extension);
+		/* The temporary file needs the proper i_extension for the compiler to do
+		 * its thing. However, create_empty_file requires the tmp_string() part to
+		 * be last, which is why the temporary file is created in two steps. */
+		char *path_stdout_tmp =
+			format("%s/%s.tmp.%s", temp_dir, input_base, tmp_string());
+		create_empty_file(path_stdout_tmp);
+		path_stdout = format("%s.%s", path_stdout_tmp, i_extension);
+		x_rename(path_stdout_tmp, path_stdout);
+		free(path_stdout_tmp);
 		add_pending_tmp_file(path_stdout);
 
 		/* run cpp on the input file to obtain the .i */
@@ -827,11 +838,6 @@ 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;
 	}
 

ccache.h

@@ -133,7 +133,7 @@ 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);

manifest.c

@@ -653,7 +653,7 @@ 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;

stats.c

@@ -126,9 +126,15 @@ 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);
 		goto end;
@@ -138,7 +144,7 @@ stats_write(const char *path, struct counters *counters)
 			fatal("Failed to write to %s", tmp_file);
 		}
 	}
-	fclose(f);
+	fclose(f); /* This also implicitly closes the fd. */
 	x_rename(tmp_file, path);
 
 end:

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");
 
@@ -418,16 +418,16 @@ get_hostname(void)
 }
 
 /*
- * Return a string to be used to distinguish temporary files. Also tries to
- * cope with NFS by adding the local hostname.
+ * Return a string to be passed to mkstemp to create a temporary file. Also
+ * tries to cope with NFS by adding the local hostname.
  */
 const char *
 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;
@@ -886,12 +886,13 @@ 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);
@@ -1126,7 +1127,10 @@ 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);