quick-and-dirty DROP backoff loop (closes #42)

package.json

@@ -15,6 +15,7 @@
   },
   "dependencies": {
     "ansi-colors": "^3.2.3",
+    "backoff": "^2.5.0",
     "bluebird": "^3.5.3",
     "cli-table": "^0.3.1",
     "debug": "^4.1.1",

src/cmd/destroy.js

@@ -1,4 +1,6 @@
+const backoff = require('backoff');
 const confirm = require('../util/confirm-prompt');
+const connectionCountTask = require('../task/connection-count');
 
 exports.command = ['destroy <target>', 'drop'];
 exports.desc = 'Destroys the given database. This cannot be undone!';
@@ -9,9 +11,55 @@ exports.builder = yargs =>
       describe: 'The database to drop. You can maintain a blacklist '
         + 'of databases to protect from this command in your .pgshrc',
       type: 'string',
+    })
+    .option('f', {
+      alias: 'fail-fast',
+      type: 'boolean',
+      describe: 'Do not wait for the database to be unused; exit immediately',
+      default: false,
     });
 
-exports.handler = async ({ target }) => {
+const fibonacciBackoff = backoff.fibonacci({
+  randomisationFactor: 0,
+  initialDelay: 300,
+  maxDelay: 12000,
+});
+
+const waitFor = (db, target, interruptHandler, failFast) =>
+  new Promise(async (resolve) => {
+    const connectionCount = connectionCountTask(db);
+    const otherConnections = await connectionCount(target);
+    const isPlural = otherConnections !== 1;
+
+    if (otherConnections === 0) {
+      return resolve();
+    }
+
+    console.log(
+      `There ${isPlural ? 'are' : 'is'} ${otherConnections} other session${isPlural ? 's' : ''}`,
+      `using the database.${failFast ? '' : ' (waiting)'}`,
+    );
+
+    if (failFast) {
+      return interruptHandler();
+    }
+
+    const readyHandler = async () => {
+      if (await connectionCount(target) > 0) {
+        fibonacciBackoff.backoff();
+      } else {
+        process.removeListener('SIGINT', interruptHandler);
+        fibonacciBackoff.removeListener('ready', readyHandler);
+        fibonacciBackoff.reset();
+        resolve();
+      }
+    };
+    process.on('SIGINT', interruptHandler);
+    fibonacciBackoff.on('ready', readyHandler);
+    return fibonacciBackoff.backoff();
+  });
+
+exports.handler = async ({ target, failFast }) => {
   const db = require('../db')();
 
   const current = db.thisDb();
@@ -33,8 +81,15 @@ exports.handler = async ({ target }) => {
     return process.exit(3);
   }
 
+  const interruptHandler = () => {
+    console.log(`\nDid not drop ${target}!`);
+    return process.exit(0);
+  };
+
   try {
+    await waitFor(db, target, interruptHandler, failFast);
     await confirm('Type the database name to drop it: ', target);
+    await waitFor(db, target, interruptHandler, failFast);
   } catch (err) {
     console.log('Not dropping.');
     return process.exit(0);
@@ -48,6 +103,6 @@ exports.handler = async ({ target }) => {
   } catch (err) {
     console.error(`Could not drop ${target}!`);
     console.error(err);
-    return process.exit(3);
+    return process.exit(4);
   }
 };

src/task/connection-count.js

@@ -0,0 +1,18 @@
+/**
+ * Returns the number of connections to the given database,
+ * without counting this one (if we're connecting to the same db).
+ */
+module.exports = db => async (databaseName) => {
+  const knex = db.connectAsSuper();
+
+  const numConnections = await knex.raw(`
+    select count(*) as connections
+    from pg_stat_activity
+    where datname = ?
+  `, [databaseName])
+    .then(({ rows }) => +rows[0].connections);
+
+  return db.thisDb() === databaseName
+    ? numConnections - 1
+    : numConnections;
+};

yarn.lock

@@ -385,6 +385,13 @@ babel-preset-jest@^24.1.0:
     "@babel/plugin-syntax-object-rest-spread" "^7.0.0"
     babel-plugin-jest-hoist "^24.1.0"
 
+backoff@^2.5.0:
+  version "2.5.0"
+  resolved "https://registry.yarnpkg.com/backoff/-/backoff-2.5.0.tgz#f616eda9d3e4b66b8ca7fca79f695722c5f8e26f"
+  integrity sha1-9hbtqdPktmuMp/ynn2lXIsX44m8=
+  dependencies:
+    precond "0.2"
+
 balanced-match@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/balanced-match/-/balanced-match-1.0.0.tgz#89b4d199ab2bee49de164ea02b89ce462d71b767"
@@ -3272,6 +3279,11 @@ postgres-interval@^1.1.0:
   dependencies:
     xtend "^4.0.0"
 
+precond@0.2:
+  version "0.2.3"
+  resolved "https://registry.yarnpkg.com/precond/-/precond-0.2.3.tgz#aa9591bcaa24923f1e0f4849d240f47efc1075ac"
+  integrity sha1-qpWRvKokkj8eD0hJ0kD0fvwQdaw=
+
 prelude-ls@~1.1.2:
   version "1.1.2"
   resolved "https://registry.yarnpkg.com/prelude-ls/-/prelude-ls-1.1.2.tgz#21932a549f5e52ffd9a827f570e04be62a97da54"