Use shm_open instead of mkstemp

shm_open is more reliable because it does not require
a writeable filesystem folder, unlike mkstemp.

client/pool-buffer.c

@@ -1,50 +1,43 @@
 #define _POSIX_C_SOURCE 200809
 #include <assert.h>
 #include <cairo.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <pango/pangocairo.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <time.h>
 #include <unistd.h>
 #include <wayland-client.h>
 #include "config.h"
 #include "pool-buffer.h"
 #include "util.h"
 
-static int create_pool_file(size_t size, char **name) {
-	static const char template[] = "sway-client-XXXXXX";
-	const char *path = getenv("XDG_RUNTIME_DIR");
-	if (path == NULL) {
-		fprintf(stderr, "XDG_RUNTIME_DIR is not set\n");
-		return -1;
-	}
-
-	size_t name_size = strlen(template) + 1 + strlen(path) + 1;
-	*name = malloc(name_size);
-	if (*name == NULL) {
-		fprintf(stderr, "allocation failed\n");
-		return -1;
-	}
-	snprintf(*name, name_size, "%s/%s", path, template);
-
-	int fd = mkstemp(*name);
-	if (fd < 0) {
-		return -1;
-	}
-
-	if (!sway_set_cloexec(fd, true)) {
-		close(fd);
-		return -1;
-	}
+static int anonymous_shm_open(void) {
+	int retries = 100;
+
+	do {
+		// try a probably-unique name
+		struct timespec ts;
+		clock_gettime(CLOCK_MONOTONIC, &ts);
+		pid_t pid = getpid();
+		char name[50];
+		snprintf(name, sizeof(name), "/sway-%x-%x",
+			(unsigned int)pid, (unsigned int)ts.tv_nsec);
+
+		// shm_open guarantees that O_CLOEXEC is set
+		int fd = shm_open(name, O_RDWR | O_CREAT | O_EXCL, 0600);
+		if (fd >= 0) {
+			shm_unlink(name);
+			return fd;
+		}
 
-	if (ftruncate(fd, size) < 0) {
-		close(fd);
-		return -1;
-	}
+		--retries;
+	} while (retries > 0 && errno == EEXIST);
 
-	return fd;
+	return -1;
 }
 
 static void buffer_release(void *data, struct wl_buffer *wl_buffer) {
@@ -62,17 +55,20 @@ static struct pool_buffer *create_buffer(struct wl_shm *shm,
 	uint32_t stride = width * 4;
 	size_t size = stride * height;
 
-	char *name;
-	int fd = create_pool_file(size, &name);
-	assert(fd != -1);
+	int fd = anonymous_shm_open();
+	if (fd == -1) {
+		return NULL;
+	}
+	if (ftruncate(fd, size) < 0) {
+		close(fd);
+		return NULL;
+	}
 	void *data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 	struct wl_shm_pool *pool = wl_shm_create_pool(shm, fd, size);
 	buf->buffer = wl_shm_pool_create_buffer(pool, 0,
 			width, height, stride, format);
 	wl_shm_pool_destroy(pool);
 	close(fd);
-	unlink(name);
-	free(name);
 
 	buf->size = size;
 	buf->width = width;