index: fix multiprocessing regression

Parallelization was broken accidentally a while ago when we switched
from git subprocesses to libgit2. Previously git subprocesses could
run in parallel within a thread pool, but libgit2 implements locking,
preventing parallel execution.

This commit:
- introduces a process pool with an individual GitRepo object for each
  worker respectively
- adds a --max_workers command-line argument

index.py

@@ -3,8 +3,9 @@
 import argparse
 import collections
 import concurrent.futures
-from concurrent.futures import ThreadPoolExecutor
+from concurrent.futures import ProcessPoolExecutor
 import hashlib
+import multiprocessing
 import sqlite3
 import sys
 
@@ -60,6 +61,8 @@ def libpath(lib):
                     choices=["sparse", "full"], default="sparse",
                     help="index mode (default: sparse)")
 parser.add_argument("-v", "--verbose", action="store_true")
+parser.add_argument("--max-workers", type=int,
+                    default=multiprocessing.cpu_count())
 args = parser.parse_args()
 
 if args.library:
@@ -90,7 +93,8 @@ def libpath(lib):
 cur = con.executescript(SCHEMA)
 
 
-def get_sourceinfos(git, lib_name, commitinfo):
+def get_sourceinfos(lib_name, commitinfo):
+    global git
     result = []
     commit_hash, commit_time, paths, _ = commitinfo
     commit_desc = git.describe(commit_hash)
@@ -114,21 +118,26 @@ def get_sourceinfos(git, lib_name, commitinfo):
     #       end='\r')
     return result
 
-def get_all_sourceinfos(git, lib_name, commitinfos):
-    result = []
-    for ci in commitinfos:
-       result += get_sourceinfos(git, lib_name, ci)
-    return result
 
-def get_all_sourceinfos_parallel(git, lib_name, commitinfos):
+def process_init(repo_path):
+    global git
+    git = GitRepo(repo_path)
+
+
+def get_all_sourceinfos_parallel(repo_path, lib_name, commitinfos,
+                                 max_workers):
     result = []
-    with ThreadPoolExecutor() as executor:
-        futures = [executor.submit(get_sourceinfos, git, lib_name, ci) for ci in commitinfos]
+    with ProcessPoolExecutor(max_workers=max_workers,
+                             initializer=process_init,
+                             initargs=(repo_path,)) as executor:
+        futures = [executor.submit(get_sourceinfos, lib_name, ci) for ci in
+                   commitinfos]
         for future in concurrent.futures.as_completed(futures):
             result += future.result()
     return result
 
-def index_full():
+
+def index_full(max_workers):
     for lib in libraries:
         print(f"Indexing library: {lib.name}")
         sys.stdout.flush()
@@ -139,7 +148,9 @@ def index_full():
         for ci in commitinfos:
             num_files += len(ci.paths)
         print(f"- found {num_files} files in {len(commitinfos)} commits")
-        sourceinfos = get_all_sourceinfos_parallel(git, lib.name, commitinfos)
+        sourceinfos = get_all_sourceinfos_parallel(libpath(lib), lib.name,
+                                                   commitinfos,
+                                                   max_workers=max_workers)
         cur = con.cursor()
         cur.execute('DELETE FROM files WHERE library = ?', (lib.name,))
         for info in sourceinfos:
@@ -149,7 +160,7 @@ def index_full():
         sys.stdout.flush()
 
 
-def index_sparse():
+def index_sparse(max_workers):
     for lib in libraries:
         print(f"Indexing library: {lib.name}")
         sys.stdout.flush()
@@ -159,7 +170,9 @@ def index_sparse():
             commitinfos = git.all_commits_with_metadata(path=p)
             print(f"- found {len(commitinfos)} versions of {p}")
             sys.stdout.flush()
-            sourceinfos += get_all_sourceinfos_parallel(git, lib.name, commitinfos)
+            sourceinfos += get_all_sourceinfos_parallel(libpath(lib), lib.name,
+                                                        commitinfos,
+                                                        max_workers=max_workers)
         print(f"- total {len(sourceinfos)} files")
         cur = con.cursor()
         cur.execute('DELETE FROM files WHERE library = ?', (lib.name,))
@@ -213,9 +226,9 @@ def prune():
 
 if not args.prune_only:
     if args.mode == 'sparse':
-        index_sparse()
+        index_sparse(args.max_workers)
     elif args.mode == 'full':
-        index_full()
+        index_full(args.max_workers)
 if not args.no_prune:
     print()
     prune()