fix: Improve CA certificate detection with fallback chain

The previous implementation incorrectly assumed empty get_ca_certs()
meant broken SSL, causing false failures in GitHub Codespaces and other
directory-based cert systems where certificates exist but aren't pre-loaded.
It would then attempt to import certifi as a workaround, but certifi wasn't
listed in requirements.txt, causing the fallback to fail with ImportError
even though the system certificates would have worked fine.

This commit replaces the naive check with a layered fallback approach that
checks multiple certificate sources. First it checks for pre-loaded system
certs (file-based systems). Then it verifies system cert paths exist
(directory-based systems like Ubuntu/Debian/Codespaces). Finally it attempts
to use certifi as an optional fallback only if needed.

This approach eliminates hard dependencies (certifi is now optional), works
in GitHub Codespaces without any setup, and fails gracefully with clear hints
for resolution when SSL is actually broken rather than failing with
ModuleNotFoundError.

Fixes #444

github_backup/github_backup.py

@@ -37,22 +37,33 @@ FNULL = open(os.devnull, "w")
 FILE_URI_PREFIX = "file://"
 logger = logging.getLogger(__name__)
 
+# Setup SSL context with fallback chain
 https_ctx = ssl.create_default_context()
-if not https_ctx.get_ca_certs():
-    import warnings
+if https_ctx.get_ca_certs():
+    # Layer 1: Certificates pre-loaded from system (file-based)
+    pass
+else:
+    paths = ssl.get_default_verify_paths()
+    if (paths.cafile and os.path.exists(paths.cafile)) or (
+        paths.capath and os.path.exists(paths.capath)
+    ):
+        # Layer 2: Cert paths exist, will be lazy-loaded on first use (directory-based)
+        pass
+    else:
+        # Layer 3: Try certifi package as optional fallback
+        try:
+            import certifi
 
-    warnings.warn(
-        "\n\nYOUR DEFAULT CA CERTS ARE EMPTY.\n"
-        + "PLEASE POPULATE ANY OF:"
-        + "".join(
-            ["\n - " + x for x in ssl.get_default_verify_paths() if type(x) is str]
-        )
-        + "\n",
-        stacklevel=2,
-    )
-    import certifi
-
-    https_ctx = ssl.create_default_context(cafile=certifi.where())
+            https_ctx = ssl.create_default_context(cafile=certifi.where())
+        except ImportError:
+            # All layers failed - no certificates available anywhere
+            sys.exit(
+                "\nERROR: No CA certificates found. Cannot connect to GitHub over SSL.\n\n"
+                "Solutions you can explore:\n"
+                "  1. pip install certifi\n"
+                "  2. Alpine: apk add ca-certificates\n"
+                "  3. Debian/Ubuntu: apt-get install ca-certificates\n\n"
+            )
 
 
 def logging_subprocess(

requirements.txt

@@ -1 +0,0 @@
-