RN: Retry `buck Fetch` in CircleCI

Summary:
Currently, various CircleCI jobs intermittently fail when trying to fetch dependencies via Maven.

This adds a few retries before failing the CircleCI job to reduce noise from intermittent network fetch failures.

Changelog:
[Internal]

Reviewed By: fkgozali

Differential Revision: D28153008

fbshipit-source-id: ac944882ff0495f568973a709d6d61ef7b51b318

scripts/circleci/buck_fetch.sh

@@ -6,8 +6,44 @@
 
 set -ex
 
-buck fetch ReactAndroid/src/test/java/com/facebook/react/modules
-buck fetch ReactAndroid/src/main/java/com/facebook/react
-buck fetch ReactAndroid/src/main/java/com/facebook/react/shell
-buck fetch ReactAndroid/src/test/...
-buck fetch ReactAndroid/src/androidTest/...
+# Source: https://gist.github.com/sj26/88e1c6584397bb7c13bd11108a579746
+# Modified to 1) address lint errors and 2) output to stderr.
+#
+# Retry a command up to a specific numer of times until it exits successfully,
+# with exponential back off.
+#
+#  $ retry 5 echo Hello
+#  Hello
+#
+#  $ retry 5 false
+#  Retry 1/5 exited 1, retrying in 1 seconds...
+#  Retry 2/5 exited 1, retrying in 2 seconds...
+#  Retry 3/5 exited 1, retrying in 4 seconds...
+#  Retry 4/5 exited 1, retrying in 8 seconds...
+#  Retry 5/5 exited 1, no more retries left.
+#
+function retry {
+  local retries=$1
+  shift
+
+  local count=0
+  until "$@"; do
+    exit=$?
+    wait=$((2 ** count))
+    count=$((count + 1))
+    if [ $count -lt "$retries" ]; then
+      echo "Retry $count/$retries exited $exit, retrying in $wait seconds..." >&2
+      sleep $wait
+    else
+      echo "Retry $count/$retries exited $exit, no more retries left." >&2
+      return $exit
+    fi
+  done
+  return 0
+}
+
+retry 3 buck fetch ReactAndroid/src/test/java/com/facebook/react/modules
+retry 3 buck fetch ReactAndroid/src/main/java/com/facebook/react
+retry 3 buck fetch ReactAndroid/src/main/java/com/facebook/react/shell
+retry 3 buck fetch ReactAndroid/src/test/...
+retry 3 buck fetch ReactAndroid/src/androidTest/...