Avoid copying temporary file when adding object to CAS in server.

The file is already a temporary file and does not need copy.  ENOSPC
is thrown during that copy in issue #609.

Fixes #678.

2 files changed, 26 insertions, 18 deletions

diff --git a/buildstream/_artifactcache/cascache.py b/buildstream/_artifactcache/cascache.py
index 3218552fb..0f7a55dc1 100644
--- a/buildstream/_artifactcache/cascache.py
+++ b/buildstream/_artifactcache/cascache.py
@@ -25,6 +25,7 @@ import os
 import stat
 import tempfile
 import uuid
+import contextlib
 from urllib.parse import urlparse
 
 import grpc
@@ -480,13 +481,14 @@ class CASCache():
     #     digest (Digest): An optional Digest object to populate
     #     path (str): Path to file to add
     #     buffer (bytes): Byte buffer to add
+    #     link_directly (bool): Whether file given by path can be linked
     #
     # Returns:
     #     (Digest): The digest of the added object
     #
     # Either `path` or `buffer` must be passed, but not both.
     #
-    def add_object(self, *, digest=None, path=None, buffer=None):
+    def add_object(self, *, digest=None, path=None, buffer=None, link_directly=False):
         # Exactly one of the two parameters has to be specified
         assert (path is None) != (buffer is None)
 
@@ -496,28 +498,34 @@ class CASCache():
         try:
             h = hashlib.sha256()
             # Always write out new file to avoid corruption if input file is modified
-            with tempfile.NamedTemporaryFile(dir=self.tmpdir) as out:
-                # Set mode bits to 0644
-                os.chmod(out.name, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
-
-                if path:
-                    with open(path, 'rb') as f:
-                        for chunk in iter(lambda: f.read(4096), b""):
-                            h.update(chunk)
-                            out.write(chunk)
+            with contextlib.ExitStack() as stack:
+                if path is not None and link_directly:
+                    tmp = stack.enter_context(open(path, 'rb'))
+                    for chunk in iter(lambda: tmp.read(4096), b""):
+                        h.update(chunk)
                 else:
-                    h.update(buffer)
-                    out.write(buffer)
+                    tmp = stack.enter_context(tempfile.NamedTemporaryFile(dir=self.tmpdir))
+                    # Set mode bits to 0644
+                    os.chmod(tmp.name, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
 
-                out.flush()
+                    if path:
+                        with open(path, 'rb') as f:
+                            for chunk in iter(lambda: f.read(4096), b""):
+                                h.update(chunk)
+                                tmp.write(chunk)
+                    else:
+                        h.update(buffer)
+                        tmp.write(buffer)
+
+                    tmp.flush()
 
                 digest.hash = h.hexdigest()
-                digest.size_bytes = os.fstat(out.fileno()).st_size
+                digest.size_bytes = os.fstat(tmp.fileno()).st_size
 
                 # Place file at final location
                 objpath = self.objpath(digest)
                 os.makedirs(os.path.dirname(objpath), exist_ok=True)
-                os.link(out.name, objpath)
+                os.link(tmp.name, objpath)
 
         except FileExistsError as e:
             # We can ignore the failed link() if the object is already in the repo.
@@ -889,7 +897,7 @@ class CASCache():
         with tempfile.NamedTemporaryFile(dir=self.tmpdir) as f:
             self._fetch_blob(remote, digest, f)
 
-            added_digest = self.add_object(path=f.name)
+            added_digest = self.add_object(path=f.name, link_directly=True)
             assert added_digest.hash == digest.hash
 
         return objpath
@@ -900,7 +908,7 @@ class CASCache():
                 f.write(data)
                 f.flush()
 
-                added_digest = self.add_object(path=f.name)
+                added_digest = self.add_object(path=f.name, link_directly=True)
                 assert added_digest.hash == digest.hash
 
     # Helper function for _fetch_directory().
diff --git a/buildstream/_artifactcache/casserver.py b/buildstream/_artifactcache/casserver.py
index f7dc89581..56e61f915 100644
--- a/buildstream/_artifactcache/casserver.py
+++ b/buildstream/_artifactcache/casserver.py
@@ -208,7 +208,7 @@ class _ByteStreamServicer(bytestream_pb2_grpc.ByteStreamServicer):
                         context.set_code(grpc.StatusCode.FAILED_PRECONDITION)
                         return response
                     out.flush()
-                    digest = self.cas.add_object(path=out.name)
+                    digest = self.cas.add_object(path=out.name, link_directly=True)
                     if digest.hash != client_digest.hash:
                         context.set_code(grpc.StatusCode.FAILED_PRECONDITION)
                         return response