util: use shm_open for in-memory files

shm_open is a POSIX function creating an in-memory file. Using it simplifies
the code and removes the dependency on XDG_RUNTIME_DIR. The only downside is
that we need to generate a random name for the shm file.

util/shm.c

@@ -0,0 +1,67 @@
+#define _POSIX_C_SOURCE 200112L
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <time.h>
+#include <unistd.h>
+#include <wlr/config.h>
+#include "util/shm.h"
+
+static void randname(char *buf) {
+	struct timespec ts;
+	clock_gettime(CLOCK_REALTIME, &ts);
+	long r = ts.tv_nsec;
+	for (int i = 0; i < 6; ++i) {
+		buf[i] = 'A'+(r&15)+(r&16)*2;
+		r >>= 5;
+	}
+}
+
+int create_shm_file(void) {
+	int retries = 100;
+	do {
+		char name[] = "/wlroots-XXXXXX";
+		randname(name + strlen(name) - 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);
+
+	return -1;
+}
+
+int allocate_shm_file(size_t size) {
+	int fd = create_shm_file();
+	if (fd < 0) {
+		return -1;
+	}
+
+#ifdef WLR_HAS_POSIX_FALLOCATE
+	int ret;
+	do {
+		ret = posix_fallocate(fd, 0, size);
+	} while (ret == EINTR);
+	if (ret != 0) {
+		close(fd);
+		errno = ret;
+		return -1;
+	}
+#else
+	int ret;
+	do {
+		ret = ftruncate(fd, size);
+	} while (ret < 0 && errno == EINTR);
+	if (ret < 0) {
+		close(fd);
+		return -1;
+	}
+#endif
+
+	return fd;
+}