Cache git status results in version module

4 files changed, 64 insertions, 53 deletions

diff --git a/libbuild2/types.hxx b/libbuild2/types.hxx
index 0005e75..e3cacce 100644
--- a/libbuild2/types.hxx
+++ b/libbuild2/types.hxx
@@ -221,6 +221,40 @@ namespace build2
   using std::thread;
   namespace this_thread = std::this_thread;
 
+  // Global, MT-safe information cache. Normally used for caching information
+  // (versions, target triplets, search paths, etc) extracted from other
+  // programs (compilers, etc).
+  //
+  // The key is normally a hash of all the inputs that can affect the output.
+  //
+  // Note that insertion is racy and it's possible the cache entry already
+  // exists, in which case we ignore our value assuming it is the same.
+  //
+  template <typename T, typename K = string>
+  class global_cache
+  {
+  public:
+    const T*
+    find (const K& k) const
+    {
+      mlock l (mutex_);
+      auto i (cache_.find (k));
+      return i != cache_.end () ? &i->second : nullptr;
+    }
+
+    const T&
+    insert (K k, T v)
+    {
+      mlock l (mutex_);
+      return cache_.insert (std::make_pair (std::move (k),
+                                            std::move (v))).first->second;
+    }
+
+  private:
+    map<K, T> cache_;
+    mutable mutex mutex_;
+  };
+
   // Exceptions.
   //
   // While <exception> is included, there is no using for std::exception --
diff --git a/libbuild2/utility.hxx b/libbuild2/utility.hxx
index c3e1921..c7ee2f4 100644
--- a/libbuild2/utility.hxx
+++ b/libbuild2/utility.hxx
@@ -535,39 +535,6 @@ namespace build2
       verbosity, pe, args, forward<F> (f), error, ignore_exit, checksum);
   }
 
-  // Global, MT-safe information cache. Normally used for caching information
-  // (versions, targets, search paths, etc) extracted from other programs
-  // (compilers, etc).
-  //
-  // The key is normally a hash of all the inputs that can affect the output.
-  //
-  // Note that insertion is racy and it's possible the cache entry already
-  // exists, in which case we ignore our value assuming it is the same.
-  //
-  template <typename T>
-  class global_cache
-  {
-  public:
-    const T*
-    find (const string& k) const
-    {
-      mlock l (mutex_);
-      auto i (cache_.find (k));
-      return i != cache_.end () ? &i->second : nullptr;
-    }
-
-    const T&
-    insert (string k, T v)
-    {
-      mlock l (mutex_);
-      return cache_.insert (make_pair (move (k), move (v))).first->second;
-    }
-
-  private:
-    map<string, T> cache_;
-    mutable mutex mutex_;
-  };
-
   // File descriptor streams.
   //
   fdpipe
diff --git a/libbuild2/version/snapshot-git.cxx b/libbuild2/version/snapshot-git.cxx
index 28d96ac..e766bc9 100644
--- a/libbuild2/version/snapshot-git.cxx
+++ b/libbuild2/version/snapshot-git.cxx
@@ -14,11 +14,20 @@ namespace build2
 {
   namespace version
   {
+    // We have to run git twice to extract the information we need and doing
+    // it repetitively is quite expensive, especially for larger repositories.
+    // So we cache it, which helps multi-package repositories.
+    //
+    static global_cache<snapshot, dir_path> cache;
+
     snapshot
-    extract_snapshot_git (const dir_path& src_root)
+    extract_snapshot_git (dir_path rep_root)
     {
+      if (const snapshot* r = cache.find (rep_root))
+        return *r;
+
       snapshot r;
-      const char* d (src_root.string ().c_str ());
+      const char* d (rep_root.string ().c_str ());
 
       // On startup git prepends the PATH environment variable value with the
       // computed directory path where its sub-programs are supposedly located
@@ -192,30 +201,31 @@ namespace build2
         // that.
       }
 
-      if (!run_finish_code (args, pr, l))
+      if (run_finish_code (args, pr, l))
+      {
+        if (r.sn == 0)
+          fail << "unable to extract git commit id/date for " << rep_root;
+
+        if (r.committed)
+        {
+          sha1 cs;
+          cs.append ("commit " + to_string (data.size ())); // Includes '\0'.
+          cs.append (data.c_str (), data.size ());
+          r.id.assign (cs.string (), 12); // 12-char abbreviated commit id.
+        }
+        else
+          r.sn++; // Add a second.
+      }
+      else
       {
         // Presumably new repository without HEAD. Return uncommitted snapshot
         // with UNIX epoch as timestamp.
         //
         r.sn = 19700101000000ULL;
         r.committed = false;
-        return r;
-      }
-
-      if (r.sn == 0)
-        fail << "unable to extract git commit id/date for " << src_root;
-
-      if (r.committed)
-      {
-        sha1 cs;
-        cs.append ("commit " + to_string (data.size ())); // Includes '\0'.
-        cs.append (data.c_str (), data.size ());
-        r.id.assign (cs.string (), 12); // 12-characters abbreviated commit id.
       }
-      else
-        r.sn++; // Add a second.
 
-      return r;
+      return cache.insert (move (rep_root), move (r));
     }
   }
 }
diff --git a/libbuild2/version/snapshot.cxx b/libbuild2/version/snapshot.cxx
index 3a885b5..d20e633 100644
--- a/libbuild2/version/snapshot.cxx
+++ b/libbuild2/version/snapshot.cxx
@@ -12,7 +12,7 @@ namespace build2
   namespace version
   {
     snapshot
-    extract_snapshot_git (const dir_path&);
+    extract_snapshot_git (dir_path);
 
     static const path git (".git");
 
@@ -46,7 +46,7 @@ namespace build2
         if (butl::entry_exists (d / git,
                                 true /* follow_symlinks */,
                                 true /* ignore_errors */))
-          return extract_snapshot_git (d);
+          return extract_snapshot_git (move (d));
       }
 
       return snapshot ();