Merge pull request #10362 from obsidiansystems/maybeLstat

Factor out `nix::maybeLstat`

(cherry-picked from commit 9b88e5284608116b7db0dbd3d5dd7a33b90d52d7)

Change-Id: Id890525e847c890fad6593c594772826ac4d1d50

src/libstore/build/local-derivation-goal.cc

@@ -2044,13 +2044,13 @@ void LocalDerivationGoal::runChild()
                             i.first, i.second.source);
 
                     std::string path = i.first;
-                    struct stat st;
+                    auto optSt = maybeLstat(path.c_str());
-                    if (lstat(path.c_str(), &st)) {
+                    if (!optSt) {
-                        if (i.second.optional && errno == ENOENT)
+                        if (i.second.optional)
                             continue;
-                        throw SysError("getting attributes of path '%s", path);
+                        throw SysError("getting attributes of required path '%s", path);
                     }
-                    if (S_ISDIR(st.st_mode))
+                    if (S_ISDIR(optSt->st_mode))
                         sandboxProfile += fmt("\t(subpath \"%s\")\n", path);
                     else
                         sandboxProfile += fmt("\t(literal \"%s\")\n", path);
@@ -2260,14 +2260,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             continue;
         }
 
-        struct stat st;
+        auto optSt = maybeLstat(actualPath.c_str());
-        if (lstat(actualPath.c_str(), &st) == -1) {
+        if (!optSt)
-            if (errno == ENOENT)
+            throw BuildError(
-                throw BuildError(
+                "builder for '%s' failed to produce output path for output '%s' at '%s'",
-                    "builder for '%s' failed to produce output path for output '%s' at '%s'",
+                worker.store.printStorePath(drvPath), outputName, actualPath);
-                    worker.store.printStorePath(drvPath), outputName, actualPath);
+        struct stat & st = *optSt;
-            throw SysError("getting attributes of path '%s'", actualPath);
-        }
 
 #ifndef __CYGWIN__
         /* Check that the output is not group or world writable, as

src/libstore/builtins/buildenv.cc

@@ -63,9 +63,9 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
             continue;
 
         else if (S_ISDIR(srcSt.st_mode)) {
-            struct stat dstSt;
+            auto dstStOpt = maybeLstat(dstFile.c_str());
-            auto res = lstat(dstFile.c_str(), &dstSt);
+            if (dstStOpt) {
-            if (res == 0) {
+                auto & dstSt = *dstStOpt;
                 if (S_ISDIR(dstSt.st_mode)) {
                     createLinks(state, srcFile, dstFile, priority);
                     continue;
@@ -81,14 +81,13 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
                     createLinks(state, srcFile, dstFile, priority);
                     continue;
                 }
-            } else if (errno != ENOENT)
+            }
-                throw SysError("getting status of '%1%'", dstFile);
         }
 
         else {
-            struct stat dstSt;
+            auto dstStOpt = maybeLstat(dstFile.c_str());
-            auto res = lstat(dstFile.c_str(), &dstSt);
+            if (dstStOpt) {
-            if (res == 0) {
+                auto & dstSt = *dstStOpt;
                 if (S_ISLNK(dstSt.st_mode)) {
                     auto prevPriority = state.priorities[dstFile];
                     if (prevPriority == priority)
@@ -103,8 +102,7 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
                         throw SysError("unlinking '%1%'", dstFile);
                 } else if (S_ISDIR(dstSt.st_mode))
                     throw Error("collision between non-directory '%1%' and directory '%2%'", srcFile, dstFile);
-            } else if (errno != ENOENT)
+            }
-                throw SysError("getting status of '%1%'", dstFile);
         }
 
         createSymlink(srcFile, dstFile);

src/libutil/util.cc

@@ -256,16 +256,22 @@ struct stat lstat(const Path & path)
     return st;
 }
 
+std::optional<struct stat> maybeLstat(const Path & path)
+{
+    std::optional<struct stat> st{std::in_place};
+    if (lstat(path.c_str(), &*st))
+    {
+        if (errno == ENOENT || errno == ENOTDIR)
+            st.reset();
+        else
+            throw SysError("getting status of '%s'", path);
+    }
+    return st;
+}
 
 bool pathExists(const Path & path)
 {
-    int res;
+    return maybeLstat(path).has_value();
-    struct stat st;
-    res = lstat(path.c_str(), &st);
-    if (!res) return true;
-    if (errno != ENOENT && errno != ENOTDIR)
-        throw SysError("getting status of %1%", path);
-    return false;
 }
 
 bool pathAccessible(const Path & path)

src/libutil/util.hh

@@ -114,6 +114,12 @@ bool isDirOrInDir(std::string_view path, std::string_view dir);
 struct stat stat(const Path & path);
 struct stat lstat(const Path & path);
 
+/**
+ * `lstat` the given path if it exists.
+ * @return std::nullopt if the path doesn't exist, or an optional containing the result of `lstat` otherwise
+ */
+std::optional<struct stat> maybeLstat(const Path & path);
+
 /**
  * @return true iff the given path exists.
  */