Store build logs in /nix/var/log/nix/drvs/<XX>

...where <XX> is the first two characters of the derivation.
Otherwise /nix/var/log/nix/drvs may become so large that we run into
all sorts of weird filesystem limits/inefficiences.  For instance,
ext3/ext4 filesystems will barf with "ext4_dx_add_entry:1551:
Directory index full!" once you hit a few million files.

1 files changed, 5 insertions, 3 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 75802c324..d9a7f3109 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2343,13 +2343,15 @@ Path DerivationGoal::openLogFile()
 {
     if (!settings.keepLog) return "";
 
+    string baseName = baseNameOf(drvPath);
+
     /* Create a log file. */
-    Path dir = (format("%1%/%2%") % settings.nixLogDir % drvsLogDir).str();
+    Path dir = (format("%1%/%2%/%3%/") % settings.nixLogDir % drvsLogDir % string(baseName, 0, 2)).str();
     createDirs(dir);
 
     if (settings.compressLog) {
 
-        Path logFileName = (format("%1%/%2%.bz2") % dir % baseNameOf(drvPath)).str();
+        Path logFileName = (format("%1%/%2%.bz2") % dir % string(baseName, 2)).str();
         AutoCloseFD fd = open(logFileName.c_str(), O_CREAT | O_WRONLY | O_TRUNC, 0666);
         if (fd == -1) throw SysError(format("creating log file `%1%'") % logFileName);
         closeOnExec(fd);
@@ -2364,7 +2366,7 @@ Path DerivationGoal::openLogFile()
         return logFileName;
 
     } else {
-        Path logFileName = (format("%1%/%2%") % dir % baseNameOf(drvPath)).str();
+        Path logFileName = (format("%1%/%2%") % dir % string(baseName, 2)).str();
         fdLogFile = open(logFileName.c_str(), O_CREAT | O_WRONLY | O_TRUNC, 0666);
         if (fdLogFile == -1) throw SysError(format("creating log file `%1%'") % logFileName);
         closeOnExec(fdLogFile);