[test] Add a test that checks for valid copyright and licenses

Tests all `.h`, `.c`, `.py`, and `Makefile` files for valid copyright
and license lines. Excludes a small number of exceptions (threading, and
divsufsort).

* Copyright does not contains `present`
* Copyright contains `Facebook, Inc`
* Copyright contains the current year
* License contains exactly the lines we expect

tests/test-license.py

@@ -0,0 +1,141 @@
+#!/usr/bin/env python3
+
+# ################################################################
+# Copyright (c) 2016-2020, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under both the BSD-style license (found in the
+# LICENSE file in the root directory of this source tree) and the GPLv2 (found
+# in the COPYING file in the root directory of this source tree).
+# You may select, at your option, one of the above-listed licenses.
+# ################################################################
+
+import datetime
+import enum
+import glob
+import os
+import sys
+
+YEAR = datetime.datetime.now().year
+
+YEAR_STR = str(YEAR)
+
+ROOT = os.path.join(os.path.dirname(__file__), "..")
+
+RELDIRS = [
+    "doc",
+    "examples",
+    "lib",
+    "programs",
+    "tests",
+]
+
+DIRS = [os.path.join(ROOT, d) for d in RELDIRS]
+
+class File(enum.Enum):
+    C = 1
+    H = 2
+    MAKE = 3
+    PY = 4
+
+SUFFIX = {
+    File.C: ".c",
+    File.H: ".h",
+    File.MAKE: "Makefile",
+    File.PY: ".py",
+}
+
+# License should certainly be in the first 10 KB.
+MAX_BYTES = 10000
+MAX_LINES = 50
+
+LICENSE_LINES = [
+    "This source code is licensed under both the BSD-style license (found in the",
+    "LICENSE file in the root directory of this source tree) and the GPLv2 (found",
+    "in the COPYING file in the root directory of this source tree).",
+    "You may select, at your option, one of the above-listed licenses.",
+]
+
+COPYRIGHT_EXCEPTIONS = {
+    # From zstdmt
+    "threading.c",
+    "threading.h",
+    # From divsufsort
+    "divsufsort.c",
+    "divsufsort.h",
+}
+
+LICENSE_EXCEPTIONS = {
+    # From divsufsort
+    "divsufsort.c",
+    "divsufsort.h",
+}
+
+
+def valid_copyright(lines):
+    for line in lines:
+        line = line.strip()
+        if "Copyright" not in line:
+            continue
+        if "present" in line:
+            return (False, f"Copyright line '{line}' contains 'present'!")
+        if "Facebook, Inc" not in line:
+            return (False, f"Copyright line '{line}' does not contain 'Facebook, Inc'")
+        if YEAR_STR not in line:
+            return (False, f"Copyright line '{line}' does not contain {YEAR}")
+        if " (c) " not in line:
+            return (False, f"Copyright line '{line}' does not contain ' (c) '!")
+        return (True, "")
+    return (False, "Copyright not found!")
+
+
+def valid_license(lines):
+    for b in range(len(lines)):
+        if LICENSE_LINES[0] not in lines[b]:
+            continue
+        for l in range(len(LICENSE_LINES)):
+            if LICENSE_LINES[l] not in lines[b + l]:
+                message = f"""Invalid license line found starting on line {b + l}!
+Expected: '{LICENSE_LINES[l]}'
+Actual: '{lines[b + l]}'"""
+                return (False, message)
+        return (True, "")
+    return (False, "License not found!")
+
+
+def valid_file(filename):
+    with open(filename, "r") as f:
+        lines = f.readlines(MAX_BYTES)
+    lines = lines[:min(len(lines), MAX_LINES)]
+                
+    ok = True
+    if os.path.basename(filename) not in COPYRIGHT_EXCEPTIONS:
+        c_ok, c_msg = valid_copyright(lines)
+        if not c_ok:
+            print(f"{filename}: {c_msg}")
+            ok = False
+    if os.path.basename(filename) not in LICENSE_EXCEPTIONS:
+        l_ok, l_msg = valid_license(lines)
+        if not l_ok:
+            print(f"{filename}: {l_msg}")
+            ok = False
+    return ok
+
+
+def main():
+    invalid_files = []
+    for directory in DIRS:
+        for suffix in SUFFIX.values():
+            files = set(glob.glob(f"{directory}/*{suffix}"))
+            files |= set(glob.glob(f"{directory}/**/*{suffix}"))
+            for filename in files:
+                if not valid_file(filename):
+                    invalid_files.append(filename)
+    if len(invalid_files) > 0:
+        print(f"Invalid files: {invalid_files}")
+    else:
+        print("Pass!")
+    return len(invalid_files)
+
+if __name__ == "__main__":
+    sys.exit(main())