Factor out nix::maybeLstat

This function is nice for more than `PosixSourceAccessor`. We can make a
few things simpler with it.

Note that the error logic slightly changes in some of the call sites, in
that we also count `ENOTDIR` and not just `ENOENT` as not having the
file, but that should be fine.

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

@@ -2053,13 +2053,13 @@ void LocalDerivationGoal::runChild()
                             i.first, i.second.source);
 
                     std::string path = i.first;
-                    struct stat st;
-                    if (lstat(path.c_str(), &st)) {
-                        if (i.second.optional && errno == ENOENT)
+                    auto optSt = maybeLstat(path.c_str());
+                    if (!optSt) {
+                        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);
@@ -2271,14 +2271,12 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             continue;
         }
 
-        struct stat st;
-        if (lstat(actualPath.c_str(), &st) == -1) {
-            if (errno == ENOENT)
-                throw BuildError(
-                    "builder for '%s' failed to produce output path for output '%s' at '%s'",
-                    worker.store.printStorePath(drvPath), outputName, actualPath);
-            throw SysError("getting attributes of path '%s'", actualPath);
-        }
+        auto optSt = maybeLstat(actualPath.c_str());
+        if (!optSt)
+            throw BuildError(
+                "builder for '%s' failed to produce output path for output '%s' at '%s'",
+                worker.store.printStorePath(drvPath), outputName, actualPath);
+        struct stat & st = *optSt;
 
 #ifndef __CYGWIN__
         /* Check that the output is not group or world writable, as

src/libstore/builtins/buildenv.cc

@@ -64,9 +64,9 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
             continue;
 
         else if (S_ISDIR(srcSt.st_mode)) {
-            struct stat dstSt;
-            auto res = lstat(dstFile.c_str(), &dstSt);
-            if (res == 0) {
+            auto dstStOpt = maybeLstat(dstFile.c_str());
+            if (dstStOpt) {
+                auto & dstSt = *dstStOpt;
                 if (S_ISDIR(dstSt.st_mode)) {
                     createLinks(state, srcFile, dstFile, priority);
                     continue;
@@ -82,14 +82,13 @@ 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 res = lstat(dstFile.c_str(), &dstSt);
-            if (res == 0) {
+            auto dstStOpt = maybeLstat(dstFile.c_str());
+            if (dstStOpt) {
+                auto & dstSt = *dstStOpt;
                 if (S_ISLNK(dstSt.st_mode)) {
                     auto prevPriority = state.priorities[dstFile];
                     if (prevPriority == priority)
@@ -104,8 +103,7 @@ 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/file-system.cc

@@ -174,15 +174,23 @@ struct stat lstat(const Path & path)
 }
 
 
+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;
-    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;
+    return maybeLstat(path).has_value();
 }
 
 bool pathAccessible(const Path & path)

src/libutil/file-system.hh

@@ -84,6 +84,7 @@ bool isDirOrInDir(std::string_view path, std::string_view dir);
  */
 struct stat stat(const Path & path);
 struct stat lstat(const Path & path);
+std::optional<struct stat> maybeLstat(const Path & path);
 
 /**
  * @return true iff the given path exists.

src/libutil/posix-source-accessor.cc

@@ -97,13 +97,7 @@ std::optional<struct stat> PosixSourceAccessor::cachedLstat(const CanonPath & pa
         if (i != cache->end()) return i->second;
     }
 
-    std::optional<struct stat> st{std::in_place};
-    if (::lstat(absPath.c_str(), &*st)) {
-        if (errno == ENOENT || errno == ENOTDIR)
-            st.reset();
-        else
-            throw SysError("getting status of '%s'", showPath(path));
-    }
+    auto st = nix::maybeLstat(absPath.c_str());
 
     auto cache(_cache.lock());
     if (cache->size() >= 16384) cache->clear();