Fix fetch-configlet script (#438)

* Fix fetch-configlet script

Shamelessly dupes @coriolinus' fix: exercism/rust#929

"Looks like Github has recently started returning non-uppercased
HTTP headers, at least some of the time. This broke the script,
which looked for a case-sensitive 'Location' header to find the
newest version. We can see this problem in spurious build failures like this."

"This fixes the script such that it no longer cares whether the initial
L is capital or not, and if it breaks again in the future, it will
give a more informative error."

* Add retry to fetch-configlet

Riffing off of @glennj's fix exercism/bash#424

This fixes the problem by adding retries to the curl attempt, combined
with the case-sensitivity fix. Also some of the logic has been reflowed.
Most notably, we now dynamically check for what archive extension to use.

bin/fetch-configlet

@@ -1,18 +1,19 @@
 #!/usr/bin/env bash
 
-LATEST=https://github.com/exercism/configlet/releases/latest
+set -eo pipefail
 
-OS=$(
-case $(uname) in
-    (Darwin*)
-        echo "mac";;
-    (Linux*)
-        echo "linux";;
-    (Windows*)
-        echo "windows";;
-    (*)
-        echo "linux";;
-esac)
+readonly RELEASES='https://github.com/exercism/configlet/releases'
+
+get_version () {
+    curl --silent --head "${RELEASES}/latest" |
+    awk -v FS=/ -v RS='\r\n' 'tolower($1) ~ /location:/ {print $NF}'
+}
+
+VERSION="$(get_version)"
+if [ -z "$VERSION" ]; then
+    echo "Latest configlet release could not be determined; aborting"
+    exit 1
+fi
 
 ARCH=$(
 case $(uname -m) in
@@ -26,7 +27,49 @@ case $(uname -m) in
         echo 64bit;;
 esac)
 
-VERSION="$(curl --head --silent $LATEST | awk -v FS=/ '/Location:/{print $NF}' | tr -d '\r')"
-URL=https://github.com/exercism/configlet/releases/download/$VERSION/configlet-$OS-${ARCH}.tgz
+OS=$(
+case $(uname) in
+    (Darwin*)
+        echo "mac";;
+    (Linux*)
+        echo "linux";;
+    (Windows*)
+        echo "windows";;
+    (*)
+        echo "linux";;
+esac)
+
+case "$OS" in
+    (windows*) EXT='zip' ;;
+    (*)        EXT='tgz' ;;
+esac
+
+URL="${RELEASES}/download/$VERSION/configlet-$OS-${ARCH}.${EXT}"
+localfile=bin/latest-configlet.${EXT}
+
+fetch_archive() {
+    local file_info=$1
+    for delay in {0..4}; do
+        sleep "$delay"
+        curl --silent --location "$URL" -o "$localfile"
+        if file "$localfile" 2>&1 | grep -q "$file_info"; then
+            # fetched successfully
+            return 0
+        fi
+    done
+    echo "Cannot fetch $URL" >&2
+    return 1
+}
+
+case "$EXT" in
+    zip)
+        fetch_archive "Zip archive data"
+        unzip "$localfile" -d bin/
+        ;;
+    tgz)
+        fetch_archive "gzip compressed data"
+        tar xzf "$localfile" -C bin/
+        ;;
+esac
 
-curl -s --location "$URL" | tar xz -C bin/
+rm -f "$localfile"