allow snapshot to be specified in make command line

Author: nikomatsakis

mk/stage0.mk

@@ -4,7 +4,9 @@ $(HBIN0_H_$(CFG_HOST_TRIPLE))/rustc$(X):		\
 		$(S)src/snapshots.txt					\
 		$(S)src/etc/get-snapshot.py $(MKFILE_DEPS)
 	@$(call E, fetch: $@)
-	$(Q)$(S)src/etc/get-snapshot.py $(CFG_HOST_TRIPLE)
+#   Note: the variable "SNAPSHOT_FILE" is generally not set, and so
+#   we generally only pass one argument to this script.  
+	$(Q)$(S)src/etc/get-snapshot.py $(CFG_HOST_TRIPLE) $(SNAPSHOT_FILE)
 	$(Q)touch $@
 
 # Host libs will be extracted by the above rule

src/etc/get-snapshot.py

@@ -3,8 +3,7 @@
 import os, tarfile, hashlib, re, shutil, sys
 from snapshot import *
 
-def unpack_snapshot(triple, snap):
-  dl_path = os.path.join(download_dir_base, snap)
+def unpack_snapshot(triple, dl_path):
   print("opening snapshot " + dl_path)
   tar = tarfile.open(dl_path)
   kernel = get_kernel(triple)
@@ -60,18 +59,27 @@ def determine_curr_snapshot(triple):
 
 # Main
 
+# this gets called with one or two arguments:
+# The first is the O/S triple.
+# The second is an optional path to the snapshot to use.
+
 triple = sys.argv[1]
-snap = determine_curr_snapshot(triple)
-dl = os.path.join(download_dir_base, snap)
-url = download_url_base + "/" + snap
-print("determined most recent snapshot: " + snap)
+if len(sys.argv) == 3:
+  dl_path = sys.argv[2]
+else:
+  snap = determine_curr_snapshot(triple)
+  dl = os.path.join(download_dir_base, snap)
+  url = download_url_base + "/" + snap
+  print("determined most recent snapshot: " + snap)
 
-if (not os.path.exists(dl)):
-  get_url_to_file(url, dl)
+  if (not os.path.exists(dl)):
+    get_url_to_file(url, dl)
 
-if (snap_filename_hash_part(snap) == hash_file(dl)):
-  print("got download with ok hash")
-else:
-  raise Exception("bad hash on download")
+  if (snap_filename_hash_part(snap) == hash_file(dl)):
+    print("got download with ok hash")
+  else:
+    raise Exception("bad hash on download")
+
+  dl_path = os.path.join(download_dir_base, snap)
 
-unpack_snapshot(triple, snap)
+unpack_snapshot(triple, dl_path)