Merge bitcoin/bitcoin#23579: script install_db4.sh check for patch command before continuing to install db4

7bb8eb0 script install_db4.sh added check for patch command (Nathan Garabedian)

Pull request description:

  First contribution.  I've read the CONTRIBUTING guide and hope I'm doing this correctly, but please kindly point out anything I should do differently.

  I found while running the contrib/install_db4.sh patch that it would fail suddenly with "patch: command not found".  I'd rather see it fail early before doing any work, allow me to install the `patch` command, and then run again.  (CentOS Linux) Here's a PR proposed to fix it.

  error message:
  ```
  ...
  db-4.8.30.NC/txn/txn_rec.c
  db-4.8.30.NC/txn/txn_region.c
  db-4.8.30.NC/txn/txn_stat.c
  db-4.8.30.NC/txn/txn_util.c
  ./contrib/install_db4.sh: line 71: patch: command not found
  ```

ACKs for top commit:
  laanwj:
    Code review and tested ACK 7bb8eb0

Tree-SHA512: f0c59ec509dff6637c41eec2923fe708456c3a7ae04495b12c5492681a1f95d1d9a643a2649df13ba8e6ac708680c627657b6989b62eff63be021e92e1d7c4a8

Author: laanwj

contrib/install_db4.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# Copyright (c) 2017-2019 The Bitcoin Core developers
+# Copyright (c) 2017-2021 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -62,6 +62,12 @@ http_get() {
   sha256_check "${3}" "${2}"
 }
 
+# Ensure the commands we use exist on the system
+if ! check_exists patch; then
+    echo "Command-line tool 'patch' not found.  Install patch and try again."
+    exit 1
+fi
+
 mkdir -p "${BDB_PREFIX}"
 http_get "${BDB_URL}" "${BDB_VERSION}.tar.gz" "${BDB_HASH}"
 tar -xzvf ${BDB_VERSION}.tar.gz -C "$BDB_PREFIX"