Respect standard TMPDIR/TEMP/TMP environment variables

If any standard temp directory environment variables are set, then don't
use the output directory as the temp directory.

Issue: #22

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

avbroot.py

@@ -211,7 +211,7 @@ def patch_subcommand(args):
 
     # Set default temp directory to the output directory because this is the
     # only way to control where external libraries put their temp files
-    tempfile.tempdir = os.path.dirname(os.path.abspath(output))
+    util.set_default_temp_dir(os.path.dirname(os.path.abspath(output)))
 
     # Decrypt keys to temp directory
     with tempfile.TemporaryDirectory(dir=util.tmpfs_path()) as key_dir:
@@ -257,7 +257,7 @@ def patch_subcommand(args):
 def extract_subcommand(args):
     # Set default temp directory to the output directory because this is the
     # only way to control where external libraries put their temp files
-    tempfile.tempdir = os.path.abspath(args.directory)
+    util.set_default_temp_dir(os.path.abspath(args.directory))
 
     with zipfile.ZipFile(args.input, 'r') as z:
         info = z.getinfo(PATH_PAYLOAD)

avbroot/util.py

@@ -57,6 +57,19 @@ def tmpfs_path():
     return None
 
 
+def set_default_temp_dir(path):
+    '''
+    Set the default temporary directory for use if the user hasn't overridden
+    it with standard environment variables.
+    '''
+
+    if any(os.getenv(k) for k in ('TMPDIR', 'TEMP', 'TMP')):
+        # Rely on tempfile's default handling
+        return
+
+    tempfile.tempdir = path
+
+
 @contextlib.contextmanager
 def open_output_file(path):
     '''