Document file metadata wrapper behavior

Co-authored-by: Andrew Kesterson <andrew@aklabs.net>

CMakeLists.txt

@@ -207,6 +207,7 @@ add_library(akstdlib SHARED
   src/stdlib.c
   src/string.c
   src/stream.c
+  src/stat.c
   src/collections.c
 )
 
@@ -317,6 +318,7 @@ set(AKSL_TESTS
     strbuf
     stream
     streamio
+    stat
     strhash
     string
     strto

include/akstdlib.h

@@ -76,6 +76,8 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <sys/stat.h>
+#include <sys/statvfs.h>
 /* off_t, for the aksl_fseeko/aksl_ftello pair. POSIX, like aksl_realpath. */
 #include <sys/types.h>
 
@@ -808,6 +810,57 @@ akerr_ErrorContext AKERR_NOIGNORE *aksl_strhash_djb2_str(const char *str, uint32
 
 /** @} */
 
+/**
+ * @brief stat(2).
+ * @param[in] pathname Path to inspect. Required.
+ * @param[out] dest File metadata. Required.
+ * @return NULL on success, an error context otherwise.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_stat(const char *pathname, struct stat *dest);
+
+/**
+ * @brief lstat(2), inspecting a symbolic link itself.
+ * @param[in] pathname Path to inspect. Required.
+ * @param[out] dest File metadata. Required.
+ * @return NULL on success, an error context otherwise.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_lstat(const char *pathname, struct stat *dest);
+
+/**
+ * @brief fstat(2).
+ * @param[in] fd Open file descriptor.
+ * @param[out] dest File metadata. Required.
+ * @return NULL on success, an error context otherwise.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_fstat(int fd, struct stat *dest);
+
+/**
+ * @brief fstatat(2).
+ * @param[in] dirfd Directory descriptor, or AT_FDCWD.
+ * @param[in] pathname Path to inspect. Required.
+ * @param[out] dest File metadata. Required.
+ * @param[in] flags libc fstatat flags, passed unchanged.
+ * @return NULL on success, an error context otherwise.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_fstatat(int dirfd, const char *pathname, struct stat *dest, int flags);
+
+/**
+ * @brief statvfs(3).
+ * @param[in] path Path on the filesystem. Required.
+ * @param[out] dest Filesystem metadata. Required.
+ * @return NULL on success, an error context otherwise.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_statvfs(const char *path, struct statvfs *dest);
+
+/**
+ * @brief fstatvfs(3).
+ * @param[in] fd Open file descriptor.
+ * @param[out] dest Filesystem metadata. Required.
+ * @return NULL on success, an error context otherwise.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_fstatvfs(int fd, struct statvfs *dest);
+
+
 /* ====================================================================== */
 /** @name Streams: open, read, write, close
  *

src/stat.c

@@ -0,0 +1,88 @@
+/* sys/stat.h and sys/statvfs.h metadata wrappers. */
+#include <akstdlib.h>
+#include <errno.h>
+#include "aksl_internal.h"
+
+/*
+ * stat(2) follows pathname through any symbolic links and writes the target's
+ * metadata into the caller-owned struct stat. A file that is gone, cannot be
+ * searched, or lives below a non-directory component is reported as the errno
+ * from libc rather than as a library-specific status.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_stat(const char *pathname, struct stat *dest)
+{
+    PREPARE_ERROR(e);
+    FAIL_ZERO_RETURN(e, pathname, AKERR_NULLPOINTER, "pathname=%p, dest=%p", (void *)pathname, (void *)dest);
+    FAIL_ZERO_RETURN(e, dest, AKERR_NULLPOINTER, "pathname=%p, dest=%p", (void *)pathname, (void *)dest);
+    errno = 0;
+    FAIL_NONZERO_RETURN(e, stat(pathname, dest), AKSL_ERRNO_OR(AKERR_IO), "pathname=%s", pathname);
+    SUCCEED_RETURN(e);
+}
+
+/*
+ * lstat(2) is stat(2) without the final symbolic-link traversal. That is the
+ * difference a caller needs when it is deciding whether a path is a link or
+ * when the link's ownership and mode are the metadata of interest.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_lstat(const char *pathname, struct stat *dest)
+{
+    PREPARE_ERROR(e);
+    FAIL_ZERO_RETURN(e, pathname, AKERR_NULLPOINTER, "pathname=%p, dest=%p", (void *)pathname, (void *)dest);
+    FAIL_ZERO_RETURN(e, dest, AKERR_NULLPOINTER, "pathname=%p, dest=%p", (void *)pathname, (void *)dest);
+    errno = 0;
+    FAIL_NONZERO_RETURN(e, lstat(pathname, dest), AKSL_ERRNO_OR(AKERR_IO), "pathname=%s", pathname);
+    SUCCEED_RETURN(e);
+}
+
+/*
+ * fstat(2) gets metadata from an already-open descriptor, so it has no path
+ * lookup race and remains useful after the file has been renamed or unlinked.
+ * A closed or otherwise invalid descriptor reports EBADF from libc.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_fstat(int fd, struct stat *dest)
+{
+    PREPARE_ERROR(e); FAIL_ZERO_RETURN(e, dest, AKERR_NULLPOINTER, "fd=%d, dest=%p", fd, (void *)dest);
+    errno = 0; FAIL_NONZERO_RETURN(e, fstat(fd, dest), AKSL_ERRNO_OR(AKERR_IO), "fd=%d", fd); SUCCEED_RETURN(e);
+}
+
+/*
+ * fstatat(2) is the directory-descriptor form of stat(2). pathname is resolved
+ * relative to dirfd unless it is absolute, and flags retain the libc choices
+ * such as inspecting a link itself. Keeping those flags unchanged prevents this
+ * wrapper from inventing a smaller policy than the POSIX call already exposes.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_fstatat(int dirfd, const char *pathname, struct stat *dest, int flags)
+{
+    PREPARE_ERROR(e);
+    FAIL_ZERO_RETURN(e, pathname, AKERR_NULLPOINTER, "dirfd=%d, pathname=%p, dest=%p", dirfd, (void *)pathname, (void *)dest);
+    FAIL_ZERO_RETURN(e, dest, AKERR_NULLPOINTER, "dirfd=%d, pathname=%p, dest=%p", dirfd, (void *)pathname, (void *)dest);
+    errno = 0;
+    FAIL_NONZERO_RETURN(e, fstatat(dirfd, pathname, dest, flags), AKSL_ERRNO_OR(AKERR_IO), "dirfd=%d pathname=%s flags=%d", dirfd, pathname, flags);
+    SUCCEED_RETURN(e);
+}
+
+/*
+ * statvfs(3) writes information about the mounted filesystem containing path,
+ * not merely the named file. The result includes the filesystem block sizes and
+ * available space the caller needs before it decides whether an operation fits.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_statvfs(const char *path, struct statvfs *dest)
+{
+    PREPARE_ERROR(e);
+    FAIL_ZERO_RETURN(e, path, AKERR_NULLPOINTER, "path=%p, dest=%p", (void *)path, (void *)dest);
+    FAIL_ZERO_RETURN(e, dest, AKERR_NULLPOINTER, "path=%p, dest=%p", (void *)path, (void *)dest);
+    errno = 0;
+    FAIL_NONZERO_RETURN(e, statvfs(path, dest), AKSL_ERRNO_OR(AKERR_IO), "path=%s", path);
+    SUCCEED_RETURN(e);
+}
+
+/*
+ * fstatvfs(3) is the descriptor form of statvfs(3). It asks the filesystem that
+ * owns fd for the same capacity and flag information without resolving a path
+ * again, and reports a bad descriptor through the errno libc supplies.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *aksl_fstatvfs(int fd, struct statvfs *dest)
+{
+    PREPARE_ERROR(e); FAIL_ZERO_RETURN(e, dest, AKERR_NULLPOINTER, "fd=%d, dest=%p", fd, (void *)dest);
+    errno = 0; FAIL_NONZERO_RETURN(e, fstatvfs(fd, dest), AKSL_ERRNO_OR(AKERR_IO), "fd=%d", fd); SUCCEED_RETURN(e);
+}

tests/test_stat.c

@@ -0,0 +1,105 @@
+/* File metadata wrapper tests. */
+#include "aksl_capture.h"
+#include <errno.h>
+#include <fcntl.h>
+
+static int test_stat_success_and_fstat(void)
+{
+    char path[AKSL_TMP_MAX];
+    int fd = -1;
+    struct stat path_dest;
+    struct stat fd_dest;
+    struct statvfs vfs_dest;
+    /* A real file makes the mode and four-byte size observable to stat(2). */
+    AKSL_CHECK(aksl_temp_file(path, sizeof(path)) == 0);
+    fd = open(path, O_RDWR);
+    AKSL_CHECK(fd >= 0);
+    AKSL_CHECK(write(fd, "stat", 4) == 4);
+    AKSL_CHECK_OK(aksl_stat(path, &path_dest));
+    AKSL_CHECK(S_ISREG(path_dest.st_mode));
+    AKSL_CHECK(path_dest.st_size == 4);
+
+    /* fstat(2) addresses the same opened inode, not a second pathname lookup. */
+    AKSL_CHECK_OK(aksl_fstat(fd, &fd_dest));
+    AKSL_CHECK(fd_dest.st_ino == path_dest.st_ino);
+
+    /* fstatvfs(3) describes the backing filesystem and has a usable block size. */
+    AKSL_CHECK_OK(aksl_fstatvfs(fd, &vfs_dest));
+    AKSL_CHECK(vfs_dest.f_frsize != 0);
+    AKSL_CHECK(close(fd) == 0);
+
+    /* A descriptor that was just closed must surface libc's EBADF. */
+    AKSL_CHECK_STATUS(aksl_fstat(fd, &fd_dest), EBADF);
+    AKSL_CHECK(unlink(path) == 0);
+    return 0;
+}
+
+static int test_stat_paths_and_fstatat(void)
+{
+    char path[AKSL_TMP_MAX];
+    char child[AKSL_TMP_MAX * 2];
+    char linkpath[AKSL_TMP_MAX * 2];
+    struct stat dest;
+    struct stat ldest;
+    struct statvfs vdest;
+    /* Missing paths and a regular file used as a directory preserve errno. */
+    AKSL_CHECK_STATUS(aksl_stat("/nonexistent/aksl/stat", &dest), ENOENT);
+    /* The temporary path is a regular file, so adding a child tests ENOTDIR. */
+    AKSL_CHECK(aksl_temp_file(path, sizeof(path)) == 0);
+    AKSL_CHECK(snprintf(child, sizeof(child), "%s/child", path) < (int)sizeof(child));
+    AKSL_CHECK_STATUS(aksl_stat(child, &dest), ENOTDIR);
+    AKSL_CHECK(snprintf(linkpath, sizeof(linkpath), "%s.link", path) < (int)sizeof(linkpath));
+    AKSL_CHECK(symlink(path, linkpath) == 0);
+
+    /* stat follows the link while lstat reports the link object itself. */
+    AKSL_CHECK_OK(aksl_stat(linkpath, &dest));
+    AKSL_CHECK_OK(aksl_lstat(linkpath, &ldest));
+    AKSL_CHECK(S_ISREG(dest.st_mode));
+    AKSL_CHECK(S_ISLNK(ldest.st_mode));
+
+    /* AT_FDCWD makes fstatat resolve this path from the current directory. */
+    AKSL_CHECK_OK(aksl_fstatat(AT_FDCWD, path, &dest, 0));
+
+    /* Invalid flags must replace stale errno with the EINVAL libc reports. */
+    errno = E2BIG;
+    AKSL_CHECK_STATUS(aksl_fstatat(AT_FDCWD, path, &dest, 0x40000000), EINVAL);
+    AKSL_CHECK(errno == EINVAL);
+
+    /* statvfs reports the filesystem containing the current directory. */
+    AKSL_CHECK_OK(aksl_statvfs(".", &vdest));
+    AKSL_CHECK(vdest.f_frsize != 0);
+    AKSL_CHECK(unlink(linkpath) == 0);
+    AKSL_CHECK(unlink(path) == 0);
+    return 0;
+}
+
+static int test_stat_null_arguments(void)
+{
+    char path[AKSL_TMP_MAX];
+    struct stat dest;
+    struct statvfs vdest;
+    /* Create a valid path so each failure below isolates a NULL argument. */
+    AKSL_CHECK(aksl_temp_file(path, sizeof(path)) == 0);
+    /* Every wrapper refuses either missing caller-owned input or output storage. */
+    AKSL_CHECK_STATUS(aksl_stat(NULL, &dest), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_stat(path, NULL), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_lstat(NULL, &dest), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_lstat(path, NULL), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_fstat(0, NULL), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_fstatat(AT_FDCWD, NULL, &dest, 0), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_fstatat(AT_FDCWD, path, NULL, 0), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_statvfs(NULL, &vdest), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_statvfs(path, NULL), AKERR_NULLPOINTER);
+    AKSL_CHECK_STATUS(aksl_fstatvfs(0, NULL), AKERR_NULLPOINTER);
+    AKSL_CHECK(unlink(path) == 0);
+    return 0;
+}
+
+int main(void)
+{
+    int failures = 0;
+    AKSL_RUN(failures, test_stat_success_and_fstat);
+    AKSL_RUN(failures, test_stat_paths_and_fstatat);
+    AKSL_RUN(failures, test_stat_null_arguments);
+    AKSL_REPORT(failures);
+}