mnt: consolidate EROFS fallbacks into errno-preserving helpers

Author: robertswiecki

mnt.cc

@@ -140,8 +140,7 @@ namespace fs = std::filesystem;
 
 static bool tryCreateDir(const std::string& path, bool log_errors = true) {
 	if (mkdir(path.c_str(), 0755) == -1 && errno != EEXIST) {
-		struct stat st;
-		if (errno != EROFS || stat(path.c_str(), &st) != 0 || !S_ISDIR(st.st_mode)) {
+		if (errno != EROFS || !util::existsAsDir(path.c_str())) {
 			if (log_errors) {
 				PLOG_D("mkdir('%s')", path.c_str());
 			}

mnt_legacy.cc

@@ -126,9 +126,7 @@ static int tryMountRW(mount_t* mpt, const char* src, const char* dst) {
 static bool createMountTarget(const std::string& path, bool is_dir) {
 	if (is_dir) {
 		if (mkdir(path.c_str(), 0711) == -1 && errno != EEXIST) {
-			struct stat st;
-			if (errno != EROFS || stat(path.c_str(), &st) != 0 ||
-			    !S_ISDIR(st.st_mode)) {
+			if (errno != EROFS || !util::existsAsDir(path.c_str())) {
 				PLOG_W("mkdir('%s')", path.c_str());
 				return false;
 			}
@@ -137,9 +135,7 @@ static bool createMountTarget(const std::string& path, bool is_dir) {
 		int fd =
 		    TEMP_FAILURE_RETRY(open(path.c_str(), O_CREAT | O_RDONLY | O_CLOEXEC, 0644));
 		if (fd == -1) {
-			struct stat st;
-			if (errno != EROFS || stat(path.c_str(), &st) != 0 ||
-			    !S_ISREG(st.st_mode)) {
+			if (errno != EROFS || !util::existsAsReg(path.c_str())) {
 				PLOG_W("open('%s', O_CREAT)", path.c_str());
 				return false;
 			}

mnt_newapi.cc

@@ -212,9 +212,7 @@ static bool createDirAt(int dir_fd, const char* path, mode_t mode) {
 		cumulative += component;
 
 		if (mkdirat(dir_fd, cumulative.c_str(), mode) == -1 && errno != EEXIST) {
-			struct stat st;
-			if (errno != EROFS || fstatat(dir_fd, cumulative.c_str(), &st, 0) != 0 ||
-			    !S_ISDIR(st.st_mode)) {
+			if (errno != EROFS || !util::existsAsDirAt(dir_fd, cumulative.c_str())) {
 				PLOG_W("mkdirat(%d, '%s')", dir_fd, cumulative.c_str());
 				return false;
 			}
@@ -433,14 +431,16 @@ static bool mountSinglePointAt(mount_t* mpt, int root_fd) {
 
 	if (mpt->is_dir) {
 		if (strcmp(rel_dst, ".") != 0 && mkdirat(root_fd, rel_dst, 0711) == -1 &&
-		    errno != EEXIST && errno != EROFS) {
-			PLOG_W("mkdirat(root_fd, '%s')", rel_dst);
+		    errno != EEXIST) {
+			if (errno != EROFS || !util::existsAsDirAt(root_fd, rel_dst)) {
+				PLOG_W("mkdirat(root_fd, '%s')", rel_dst);
+			}
 		}
 	} else {
 		int fd = openat(root_fd, rel_dst, O_CREAT | O_RDONLY | O_CLOEXEC, 0644);
 		if (fd >= 0) {
 			close(fd);
-		} else if (errno != EROFS) {
+		} else if (errno != EROFS || !util::existsAsRegAt(root_fd, rel_dst)) {
 			PLOG_W("openat(root_fd, '%s', O_CREAT)", rel_dst);
 		}
 	}

util.cc

@@ -225,9 +225,7 @@ bool createDirRecursively(const char* dir) {
 		*next = '\0';
 
 		if (mkdirat(prev_dir_fd, curr, 0755) == -1 && errno != EEXIST) {
-			struct stat st;
-			if (errno != EROFS || fstatat(prev_dir_fd, curr, &st, 0) != 0 ||
-			    !S_ISDIR(st.st_mode)) {
+			if (errno != EROFS || !util::existsAsDirAt(prev_dir_fd, curr)) {
 				PLOG_W("mkdir(%s, 0755)", QC(curr));
 				close(prev_dir_fd);
 				return false;

util.h

@@ -22,11 +22,13 @@
 #ifndef NS_UTIL_H
 #define NS_UTIL_H
 
+#include <errno.h>
 #include <inttypes.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <sys/resource.h>
+#include <sys/stat.h>
 
 #include <string>
 #include <vector>
@@ -80,6 +82,54 @@ bool makeRangeCOE(unsigned int first, unsigned int last);
 const char* stripLeadingSlashes(const char* path);
 bool kernelVersionAtLeast(int major, int minor, int patch);
 
+/*
+ * EROFS fallback helpers for read-only filesystems (e.g. NFS).
+ *
+ * On NFSv3/v4 with a cold dentry cache, mkdir()/open(O_CREAT) can return
+ * EROFS instead of EEXIST for entries that already exist.  These helpers
+ * verify the entry exists with the expected type, preserving the original
+ * errno for accurate PLOG logging on failure.
+ */
+inline bool existsAsDir(const char* path) {
+	int saved = errno;
+	struct stat st;
+	if (stat(path, &st) == 0 && S_ISDIR(st.st_mode)) {
+		return true;
+	}
+	errno = saved;
+	return false;
+}
+
+inline bool existsAsDirAt(int dir_fd, const char* path) {
+	int saved = errno;
+	struct stat st;
+	if (fstatat(dir_fd, path, &st, 0) == 0 && S_ISDIR(st.st_mode)) {
+		return true;
+	}
+	errno = saved;
+	return false;
+}
+
+inline bool existsAsReg(const char* path) {
+	int saved = errno;
+	struct stat st;
+	if (stat(path, &st) == 0 && S_ISREG(st.st_mode)) {
+		return true;
+	}
+	errno = saved;
+	return false;
+}
+
+inline bool existsAsRegAt(int dir_fd, const char* path) {
+	int saved = errno;
+	struct stat st;
+	if (fstatat(dir_fd, path, &st, 0) == 0 && S_ISREG(st.st_mode)) {
+		return true;
+	}
+	errno = saved;
+	return false;
+}
+
 }  // namespace util
 
 #endif /* NS_UTIL_H */