From 4531585275254f13dae1ff61434e15865a1e796a Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Mon, 29 Jan 2024 17:16:18 -0500
Subject: [PATCH] Factor out the Unix-specific parts of `canonPathInner`

This prepares the code to also support Windows paths in the next commit.
---
 src/libutil/canon-path.cc     |  2 +-
 src/libutil/file-path-impl.hh | 52 +++++++++++++++++++++++++++++------
 src/libutil/file-system.cc    |  2 +-
 3 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/src/libutil/canon-path.cc b/src/libutil/canon-path.cc
index fcd53862b..27f048697 100644
--- a/src/libutil/canon-path.cc
+++ b/src/libutil/canon-path.cc
@@ -8,7 +8,7 @@ CanonPath CanonPath::root = CanonPath("/");
 
 static std::string absPathPure(std::string_view path)
 {
-    return canonPathInner(path, [](auto &, auto &){});
+    return canonPathInner<UnixPathTrait>(path, [](auto &, auto &){});
 }
 
 CanonPath::CanonPath(std::string_view raw)
diff --git a/src/libutil/file-path-impl.hh b/src/libutil/file-path-impl.hh
index 39159c7c2..941d433e0 100644
--- a/src/libutil/file-path-impl.hh
+++ b/src/libutil/file-path-impl.hh
@@ -10,6 +10,39 @@
 
 namespace nix {
 
+/**
+ * Unix-style path primives.
+ *
+ * Nix'result own "logical" paths are always Unix-style. So this is always
+ * used for that, and additionally used for native paths on Unix.
+ */
+struct UnixPathTrait
+{
+    using CharT = char;
+
+    using String = std::string;
+
+    using StringView = std::string_view;
+
+    constexpr static char preferredSep = '/';
+
+    static inline bool isPathSep(char c)
+    {
+        return c == '/';
+    }
+
+    static inline size_t findPathSep(StringView path, size_t from = 0)
+    {
+        return path.find('/', from);
+    }
+
+    static inline size_t rfindPathSep(StringView path, size_t from = StringView::npos)
+    {
+        return path.rfind('/', from);
+    }
+};
+
+
 /**
  * Core pure path canonicalization algorithm.
  *
@@ -24,25 +57,26 @@ namespace nix {
  *   This is a chance to modify those two paths in arbitrary way, e.g. if
  *   "result" points to a symlink.
  */
-typename std::string canonPathInner(
-    std::string_view remaining,
+template<class PathDict>
+typename PathDict::String canonPathInner(
+    typename PathDict::StringView remaining,
     auto && hookComponent)
 {
     assert(remaining != "");
 
-    std::string result;
+    typename PathDict::String result;
     result.reserve(256);
 
     while (true) {
 
         /* Skip slashes. */
-        while (!remaining.empty() && remaining[0] == '/')
+        while (!remaining.empty() && PathDict::isPathSep(remaining[0]))
             remaining.remove_prefix(1);
 
         if (remaining.empty()) break;
 
         auto nextComp = ({
-            auto nextPathSep = remaining.find('/');
+            auto nextPathSep = PathDict::findPathSep(remaining);
             nextPathSep == remaining.npos ? remaining : remaining.substr(0, nextPathSep);
         });
 
@@ -53,14 +87,14 @@ typename std::string canonPathInner(
         /* If `..', delete the last component. */
         else if (nextComp == "..")
         {
-            if (!result.empty()) result.erase(result.rfind('/'));
+            if (!result.empty()) result.erase(PathDict::rfindPathSep(result));
             remaining.remove_prefix(2);
         }
 
         /* Normal component; copy it. */
         else {
-            result += '/';
-            if (const auto slash = remaining.find('/'); slash == result.npos) {
+            result += PathDict::preferredSep;
+            if (const auto slash = PathDict::findPathSep(remaining); slash == result.npos) {
                 result += remaining;
                 remaining = {};
             } else {
@@ -73,7 +107,7 @@ typename std::string canonPathInner(
     }
 
     if (result.empty())
-        result = "/";
+        result = typename PathDict::String { PathDict::preferredSep };
 
     return result;
 }
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index 3c019a9ed..ff83bc4ea 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -78,7 +78,7 @@ Path canonPath(PathView path, bool resolveSymlinks)
        arbitrary (but high) limit to prevent infinite loops. */
     unsigned int followCount = 0, maxFollow = 1024;
 
-    return canonPathInner(
+    return canonPathInner<UnixPathTrait>(
         path,
         [&followCount, &temp, maxFollow, resolveSymlinks]
         (std::string & result, std::string_view & remaining) {