Add flock_mmap

5 files changed, 239 insertions, 0 deletions

diff --git a/flock_mmap/.gitignore b/flock_mmap/.gitignore
new file mode 100644
index 0000000..052f66c
--- /dev/null
+++ b/flock_mmap/.gitignore
@@ -0,0 +1,2 @@
+flock_mmap
+lock
diff --git a/flock_mmap/Makefile b/flock_mmap/Makefile
new file mode 100644
index 0000000..0275f91
--- /dev/null
+++ b/flock_mmap/Makefile
@@ -0,0 +1,15 @@
+CC := cc
+OBJ = flock_mmap lock
+
+do: build test
+
+clean:
+	rm -f $(OBJ)
+
+flock_mmap: flock_mmap.c
+	$(CC) -std=c11 -Wall -Wextra -g -O0 -o flock_mmap flock_mmap.c
+
+build: flock_mmap
+
+test: build
+	./tests.sh
diff --git a/flock_mmap/README.md b/flock_mmap/README.md
new file mode 100644
index 0000000..623fd5c
--- /dev/null
+++ b/flock_mmap/README.md
@@ -0,0 +1,50 @@
+# Mixing flock() and close() on Various Unices
+https://stackoverflow.com/questions/70045539/file-retains-lock-after-mmap-and-close
+
+This turned out to be a bigger problem than I thought. Well, too late to fix the
+kernel now.
+
+Added `fcntl()` as control.
+
+## INSTALL
+```sh
+mkdir flock_mmap
+cd flock_mmap
+
+wget \
+	https://github.com/dxdxdt/gists/raw/master/flock_mmap/tests.sh \
+	https://github.com/dxdxdt/gists/raw/master/flock_mmap/Makefile \
+	https://github.com/dxdxdt/gists/raw/master/flock_mmap/flock_mmap.c \
+	https://github.com/dxdxdt/gists/raw/master/flock_mmap/README.md
+
+chmod 755 tests.sh
+```
+
+## Run it
+```sh
+make 2> /dev/null
+```
+
+### Result A
+- Linux
+
+```
+./tests.sh
+[TEST]fcntl(fd, F_SETLK, ...) and close() unlocks the file: YES
+[TEST]fcntl(fd, F_SETLK, ...) and holding unlocks the file: NO
+[TEST]flock() and close() unlocks the file*: NO
+[TEST]flock() and holding unlocks the file : NO
+```
+
+### Result B
+- FreeBSD
+- OpenBSD
+- mac
+
+```
+./tests.sh
+[TEST]fcntl(fd, F_SETLK, ...) and close() unlocks the file: YES
+[TEST]fcntl(fd, F_SETLK, ...) and holding unlocks the file: NO
+[TEST]flock() and close() unlocks the file*: YES
+[TEST]flock() and holding unlocks the file : NO
+```
diff --git a/flock_mmap/flock_mmap.c b/flock_mmap/flock_mmap.c
new file mode 100644
index 0000000..7954a40
--- /dev/null
+++ b/flock_mmap/flock_mmap.c
@@ -0,0 +1,118 @@
+#include <stdio.h>
+#include <stdbool.h>
+#include <string.h>
+
+#include <unistd.h>
+#include <getopt.h>
+#include <fcntl.h>
+#include <sys/file.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+
+static const size_t M_SIZE = 1;
+static bool use_fcntl = false;
+static bool close_fd = true;
+static bool should_wait = false;
+static bool verbose = false;
+static const char *path;
+
+static bool parse_args (const int argc, const char **argv) {
+	int opt;
+
+	while ((opt = getopt(argc, (char *const*)argv, "fnwV")) != -1) {
+		switch (opt) {
+		case 'f': use_fcntl = true; break;
+		case 'n': close_fd = false; break;
+		case 'w': should_wait = true; break;
+		case 'V': verbose = true; break;
+		default: return false;
+		}
+	}
+
+	if (optind >= argc) {
+		return false;
+	}
+	else {
+		path = argv[optind];
+	}
+
+	return true;
+}
+
+int main (const int argc, const char **argv) {
+	int ec = 0;
+	int fd = -1;
+	int f_ret;
+	void *m = MAP_FAILED;
+
+	if (!parse_args(argc, argv)) {
+		ec = 2;
+		fprintf(stderr, "Usage: %s -fnw <lock file path>\n", argv[0]);
+		goto END;
+	}
+
+	if (!verbose) {
+		close(STDOUT_FILENO);
+	}
+
+	fd = open(path, O_CREAT | O_TRUNC | O_RDWR, 0755);
+	if (fd < 0) {
+		ec = 1;
+		perror(path);
+		goto END;
+	}
+
+	if (use_fcntl) {
+		struct flock fl;
+
+		memset(&fl, 0, sizeof(fl));
+		fl.l_type = F_WRLCK;
+		fl.l_whence = SEEK_SET;
+		fl.l_len = M_SIZE;
+
+		f_ret = fcntl(fd, F_SETLK, &fl);
+	}
+	else {
+		f_ret = flock(fd, LOCK_EX | LOCK_NB);
+	}
+
+	if (f_ret != 0) {
+		ec = 1;
+		perror(path);
+		goto END;
+	}
+
+	f_ret = ftruncate(fd, M_SIZE);
+	if (f_ret != 0) {
+		ec = 1;
+		perror(path);
+		goto END;
+	}
+
+	m = mmap(NULL, M_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (m == MAP_FAILED) {
+		ec = 1;
+		perror(path);
+		goto END;
+	}
+
+	// This block
+	if (close_fd) {
+		close(fd);
+		fd = -1;
+	}
+
+	printf("Lock holding\n");
+	if (should_wait) {
+		pause();
+	}
+
+END:
+	if (fd >= 0) {
+		close(fd);
+	}
+	if (m != MAP_FAILED) {
+		munmap(m, M_SIZE);
+	}
+	return ec;
+}
diff --git a/flock_mmap/tests.sh b/flock_mmap/tests.sh
new file mode 100755
index 0000000..4cad797
--- /dev/null
+++ b/flock_mmap/tests.sh
@@ -0,0 +1,54 @@
+#!/bin/sh
+LOCK_F="lock"
+
+setyes () {
+	echo "[TEST]$TEST_NAME: YES"
+}
+
+setno () {
+	echo "[TEST]$TEST_NAME: NO"
+}
+
+cleanup () {
+	kill -TERM $!
+}
+
+TEST_NAME="fcntl(fd, F_SETLK, ...) and close() unlocks the file"
+./flock_mmap -fw "$LOCK_F" & sleep 1
+if ./flock_mmap -f "$LOCK_F"
+then
+	setyes
+else
+	setno
+fi
+cleanup
+
+TEST_NAME="fcntl(fd, F_SETLK, ...) and holding unlocks the file"
+./flock_mmap -fwn "$LOCK_F" & sleep 1
+if ./flock_mmap -f "$LOCK_F"
+then
+	setyes
+else
+	setno
+fi
+cleanup
+
+TEST_NAME="flock() and close() unlocks the file*"
+./flock_mmap -w "$LOCK_F" & sleep 1
+if ./flock_mmap "$LOCK_F"
+then
+	setyes
+else
+	setno
+fi
+cleanup
+
+TEST_NAME="flock() and holding unlocks the file "
+./flock_mmap -wn "$LOCK_F" & sleep 1
+if ./flock_mmap "$LOCK_F"
+then
+	setyes
+else
+	setno
+fi
+cleanup