S3BinaryCacheStore:: Eliminate a string copy while uploading

This cuts hydra-queue-runner's peak memory usage by about a third.

2 files changed, 15 insertions, 1 deletions

diff --git a/src/libutil/hash.cc b/src/libutil/hash.cc
index 81aced0fde16..49e781980f3a 100644
--- a/src/libutil/hash.cc
+++ b/src/libutil/hash.cc
@@ -106,7 +106,7 @@ Hash parseHash(HashType ht, const string & s)
         string s2(s, i * 2, 2);
         if (!isxdigit(s2[0]) || !isxdigit(s2[1]))
             throw BadHash(format("invalid hash ‘%1%’") % s);
-        std::istringstream str(s2);
+        istringstream_nocopy str(s2);
         int n;
         str >> std::hex >> n;
         hash.hash[i] = n;
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 50b96f7ed92c..a8f6f99b957f 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -431,4 +431,18 @@ void callSuccess(
 }
 
 
+/* A variant of std::istringstream that doesn't its string
+   argument. This is useful for large strings. The caller must ensure
+   that the string object is not destroyed while it's referenced by
+   this object. */
+struct istringstream_nocopy : public std::stringstream
+{
+    istringstream_nocopy(const std::string & s)
+    {
+        rdbuf()->pubsetbuf(
+            (char *) s.data(), s.size());
+    }
+};
+
+
 }