From 20a1a65d377cb49dd0a1b60a07623760bfc730d1 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 21 May 2019 14:55:43 +0200
Subject: [PATCH] Only rewrite the lockfile if it changed

This removes spurious warnings about failure to write the lockfile.
---
 src/libexpr/primops/flake.cc | 28 ++++++++++++++++------------
 src/libexpr/primops/flake.hh | 21 +++++++++++++++++++++
 2 files changed, 37 insertions(+), 12 deletions(-)

diff --git a/src/libexpr/primops/flake.cc b/src/libexpr/primops/flake.cc
index 3cbb0c1ef..788977c72 100644
--- a/src/libexpr/primops/flake.cc
+++ b/src/libexpr/primops/flake.cc
@@ -465,31 +465,35 @@ ResolvedFlake resolveFlakeFromLockFile(EvalState & state, const FlakeRef & flake
 ResolvedFlake resolveFlake(EvalState & state, const FlakeRef & topRef, HandleLockFile handleLockFile)
 {
     Flake flake = getFlake(state, topRef, allowedToUseRegistries(handleLockFile, true));
-    LockFile lockFile;
+    LockFile oldLockFile;
 
     if (!recreateLockFile (handleLockFile)) {
         // If recreateLockFile, start with an empty lockfile
-        lockFile = readLockFile(flake.storePath + "/flake.lock"); // FIXME: symlink attack
+        oldLockFile = readLockFile(flake.storePath + "/flake.lock"); // FIXME: symlink attack
     }
 
+    LockFile lockFile(oldLockFile);
+
     ResolvedFlake resFlake = resolveFlakeFromLockFile(state, topRef, handleLockFile, lockFile, true);
     lockFile = entryToLockFile(dependenciesToFlakeEntry(resFlake));
 
-    if (allowedToWrite(handleLockFile)) {
-        if (auto refData = std::get_if<FlakeRef::IsPath>(&topRef.data)) {
-            writeLockFile(lockFile, refData->path + (topRef.subdir == "" ? "" : "/" + topRef.subdir) + "/flake.lock");
+    if (!(lockFile == oldLockFile)) {
+        if (allowedToWrite(handleLockFile)) {
+            if (auto refData = std::get_if<FlakeRef::IsPath>(&topRef.data)) {
+                writeLockFile(lockFile, refData->path + (topRef.subdir == "" ? "" : "/" + topRef.subdir) + "/flake.lock");
 
-            // Hack: Make sure that flake.lock is visible to Git, so it ends up in the Nix store.
-            runProgram("git", true, { "-C", refData->path, "add",
-                      (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock" });
-        } else std::cout << "Cannot write lockfile because the FlakeRef isn't of the form IsPath." << std::endl;
-    } else if (handleLockFile != AllPure && handleLockFile != TopRefUsesRegistries)
-        std::cout << "Using updating lockfile without writing it to file" << std::endl;
+                // Hack: Make sure that flake.lock is visible to Git, so it ends up in the Nix store.
+                runProgram("git", true, { "-C", refData->path, "add",
+                                          (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock" });
+            } else std::cout << "Cannot write lockfile because the FlakeRef isn't of the form IsPath." << std::endl;
+        } else if (handleLockFile != AllPure && handleLockFile != TopRefUsesRegistries)
+            std::cout << "Using updating lockfile without writing it to file" << std::endl;
+    }
 
     return resFlake;
 }
 
-void updateLockFile (EvalState & state, const FlakeUri & flakeUri, bool recreateLockFile)
+void updateLockFile(EvalState & state, const FlakeUri & flakeUri, bool recreateLockFile)
 {
     FlakeRef flakeRef(flakeUri);
     resolveFlake(state, flakeRef, recreateLockFile ? RecreateLockFile : UpdateLockFile);
diff --git a/src/libexpr/primops/flake.hh b/src/libexpr/primops/flake.hh
index 6f91686a6..e43b860ee 100644
--- a/src/libexpr/primops/flake.hh
+++ b/src/libexpr/primops/flake.hh
@@ -24,6 +24,11 @@ struct LockFile
         FlakeRef ref;
         Hash contentHash;
         NonFlakeEntry(const FlakeRef & flakeRef, const Hash & hash) : ref(flakeRef), contentHash(hash) {};
+
+        bool operator ==(const NonFlakeEntry & other) const
+        {
+            return ref == other.ref && contentHash == other.contentHash;
+        }
     };
 
     struct FlakeEntry
@@ -33,10 +38,26 @@ struct LockFile
         std::map<FlakeRef, FlakeEntry> flakeEntries;
         std::map<FlakeAlias, NonFlakeEntry> nonFlakeEntries;
         FlakeEntry(const FlakeRef & flakeRef, const Hash & hash) : ref(flakeRef), contentHash(hash) {};
+
+        bool operator ==(const FlakeEntry & other) const
+        {
+            return
+                ref == other.ref
+                && contentHash == other.contentHash
+                && flakeEntries == other.flakeEntries
+                && nonFlakeEntries == other.nonFlakeEntries;
+        }
     };
 
     std::map<FlakeRef, FlakeEntry> flakeEntries;
     std::map<FlakeAlias, NonFlakeEntry> nonFlakeEntries;
+
+    bool operator ==(const LockFile & other) const
+    {
+        return
+            flakeEntries == other.flakeEntries
+            && nonFlakeEntries == other.nonFlakeEntries;
+    }
 };
 
 typedef std::vector<std::shared_ptr<FlakeRegistry>> Registries;