`absPath`: just take a `std::string_view`

1. Slightly more efficient

2. Easier to call

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

src/libutil/canon-path.cc

@@ -6,11 +6,11 @@ namespace nix {
 CanonPath CanonPath::root = CanonPath("/");
 
 CanonPath::CanonPath(std::string_view raw)
-    : path(absPath((Path) raw, "/"))
+    : path(absPath(raw, "/"))
 { }
 
 CanonPath::CanonPath(std::string_view raw, const CanonPath & root)
-    : path(absPath((Path) raw, root.abs()))
+    : path(absPath(raw, root.abs()))
 { }
 
 CanonPath::CanonPath(const std::vector<std::string> & elems)
@@ -22,7 +22,7 @@ CanonPath::CanonPath(const std::vector<std::string> & elems)
 
 CanonPath CanonPath::fromCwd(std::string_view path)
 {
-    return CanonPath(unchecked_t(), absPath((Path) path));
+    return CanonPath(unchecked_t(), absPath(path));
 }
 
 std::optional<CanonPath> CanonPath::parent() const

src/libutil/file-system.cc

@@ -21,9 +21,16 @@ namespace fs = std::filesystem;
 
 namespace nix {
 
-Path absPath(Path path, std::optional<PathView> dir, bool resolveSymlinks)
+Path absPath(PathView path, std::optional<PathView> dir, bool resolveSymlinks)
 {
+    std::string scratch;
+
     if (path[0] != '/') {
+        // In this case we need to call `canonPath` on a newly-created
+        // string. We set `scratch` to that string first, and then set
+        // `path` to `scratch`. This ensures the newly-created string
+        // lives long enough for the call to `canonPath`, and allows us
+        // to just accept a `std::string_view`.
         if (!dir) {
 #ifdef __GNU__
             /* GNU (aka. GNU/Hurd) doesn't have any limitation on path
@@ -35,12 +42,13 @@ Path absPath(Path path, std::optional<PathView> dir, bool resolveSymlinks)
             if (!getcwd(buf, sizeof(buf)))
 #endif
                 throw SysError("cannot get cwd");
-            path = concatStrings(buf, "/", path);
+            scratch = concatStrings(buf, "/", path);
 #ifdef __GNU__
             free(buf);
 #endif
         } else
-            path = concatStrings(*dir, "/", path);
+            scratch = concatStrings(*dir, "/", path);
+        path = scratch;
     }
     return canonPath(path, resolveSymlinks);
 }

src/libutil/file-system.hh

@@ -41,7 +41,7 @@ struct Source;
  * specified directory, or the current directory otherwise.  The path
  * is also canonicalised.
  */
-Path absPath(Path path,
+Path absPath(PathView path,
     std::optional<PathView> dir = {},
     bool resolveSymlinks = false);