util/shm: add allocate_shm_file_pair

This function behaves like allocate_shm_file, except it also
returns a read-only FD. This is useful to share the same segment
of memory with many Wayland clients.

include/util/shm.h

@@ -1,7 +1,10 @@
 #ifndef UTIL_SHM_H
 #define UTIL_SHM_H
 
+#include <stdbool.h>
+
 int create_shm_file(void);
 int allocate_shm_file(size_t size);
+bool allocate_shm_file_pair(size_t size, int *rw_fd, int *ro_fd);
 
 #endif

util/shm.c

@@ -8,6 +8,8 @@
 #include <wlr/config.h>
 #include "util/shm.h"
 
+#define RANDNAME_PATTERN "/wlroots-XXXXXX"
+
 static void randname(char *buf) {
 	struct timespec ts;
 	clock_gettime(CLOCK_REALTIME, &ts);
@@ -18,17 +20,15 @@ static void randname(char *buf) {
 	}
 }
 
-int create_shm_file(void) {
+static int excl_shm_open(char *name) {
 	int retries = 100;
 	do {
-		char name[] = "/wlroots-XXXXXX";
-		randname(name + strlen(name) - 6);
+		randname(name + strlen(RANDNAME_PATTERN) - 6);
 
 		--retries;
 		// CLOEXEC is guaranteed to be set by shm_open
 		int fd = shm_open(name, O_RDWR | O_CREAT | O_EXCL, 0600);
 		if (fd >= 0) {
-			shm_unlink(name);
 			return fd;
 		}
 	} while (retries > 0 && errno == EEXIST);
@@ -37,10 +37,12 @@ int create_shm_file(void) {
 }
 
 int allocate_shm_file(size_t size) {
-	int fd = create_shm_file();
+	char name[] = RANDNAME_PATTERN;
+	int fd = excl_shm_open(name);
 	if (fd < 0) {
 		return -1;
 	}
+	shm_unlink(name);
 
 	int ret;
 	do {
@@ -53,3 +55,35 @@ int allocate_shm_file(size_t size) {
 
 	return fd;
 }
+
+bool allocate_shm_file_pair(size_t size, int *rw_fd_ptr, int *ro_fd_ptr) {
+	char name[] = RANDNAME_PATTERN;
+	int rw_fd = excl_shm_open(name);
+	if (rw_fd < 0) {
+		return false;
+	}
+
+	// CLOEXEC is guaranteed to be set by shm_open
+	int ro_fd = shm_open(name, O_RDONLY, 0);
+	if (ro_fd < 0) {
+		shm_unlink(name);
+		close(rw_fd);
+		return false;
+	}
+
+	shm_unlink(name);
+
+	int ret;
+	do {
+		ret = ftruncate(rw_fd, size);
+	} while (ret < 0 && errno == EINTR);
+	if (ret < 0) {
+		close(rw_fd);
+		close(ro_fd);
+		return false;
+	}
+
+	*rw_fd_ptr = rw_fd;
+	*ro_fd_ptr = ro_fd;
+	return true;
+}