whisper-server: validate model downloads (size + ggml magic), self-heal poisoned files

The first server deployment downloaded only 1101 bytes (a redirect/error
page) and the entrypoint promoted it to ggml-large-v3.bin, after which
every restart skipped the download and the server ran without a valid
model. Now both fresh downloads and existing files are validated
(minimum 50 MB + the ggml magic bytes); invalid files are logged,
deleted, and re-downloaded, and a failed download dumps the first 400
bytes of what was actually received for diagnosis before exiting.

Validated locally: a poisoned model file is detected, removed, and a
valid one re-downloaded; inference served correctly afterwards.

server/whisper-server/entrypoint.sh

@@ -2,6 +2,11 @@
 # Downloads the configured GGML model on first start, then runs whisper-server.
 # Uses the GPU (Vulkan) automatically when one is present; set NO_GPU=1
 # to force CPU.
+#
+# Both the freshly downloaded file and any existing file are validated
+# (minimum size + ggml magic bytes) so a truncated download or a saved
+# error/redirect page can never be mistaken for a model — a poisoned
+# file is deleted and re-downloaded instead.
 set -eu
 
 MODEL="${MODEL:-small}"
@@ -10,14 +15,43 @@ HOST="${HOST:-0.0.0.0}"
 PORT="${PORT:-8085}"
 MODEL_DIR="${MODEL_DIR:-/models}"
 FILE="$MODEL_DIR/ggml-$MODEL.bin"
+URL="https://huggingface.co/ggerganov/whisper.cpp/resolve/main/ggml-$MODEL.bin"
+
+# smallest supported model (ggml-tiny.bin) is ~75 MB
+MIN_SIZE=50000000
+GGML_MAGIC="$(printf 'lmgg')"   # GGML_FILE_MAGIC 0x67676d6c, little-endian
+
+is_valid_model() {
+    [ -s "$1" ] || return 1
+    [ "$(stat -c%s "$1")" -ge "$MIN_SIZE" ] || return 1
+    [ "$(head -c 4 "$1")" = "$GGML_MAGIC" ] || return 1
+    return 0
+}
 
 mkdir -p "$MODEL_DIR"
-if [ ! -s "$FILE" ]; then
-    echo "downloading ggml-$MODEL.bin to $FILE ..."
-    curl -fL --retry 3 -C - -o "$FILE.part" \
-        "https://huggingface.co/ggerganov/whisper.cpp/resolve/main/ggml-$MODEL.bin"
+
+if ! is_valid_model "$FILE" && [ -e "$FILE" ]; then
+    echo "existing $FILE is invalid ($(stat -c%s "$FILE") bytes) — " \
+        "deleting and re-downloading"
+fi
+
+if ! is_valid_model "$FILE"; then
+    rm -f "$FILE" "$FILE.part"
+    echo "downloading $URL to $FILE ..."
+    curl -fLS --retry 3 --retry-delay 5 -C - -o "$FILE.part" "$URL"
+
+    if ! is_valid_model "$FILE.part"; then
+        size="$(stat -c%s "$FILE.part" 2>/dev/null || echo 0)"
+        echo "ERROR: download is not a valid model ($size bytes)."
+        echo "------- first 400 bytes of the response (probably an error/redirect page):"
+        head -c 400 "$FILE.part"
+        echo ""
+        echo "------- end of response"
+        rm -f "$FILE.part"
+        exit 1
+    fi
     mv "$FILE.part" "$FILE"
-    echo "done."
+    echo "downloaded $(stat -c%s "$FILE") bytes, magic ok."
 fi
 
 GPU_FLAGS=""