Fail the build when a prepare target's variables don't match its run query

.changeset/prepare-variables.md

@@ -0,0 +1,5 @@
+---
+"@shopify/shopify_function": minor
+---
+
+Fail the build when a JavaScript function's prepare target returns variables its run query doesn't accept, and warn about variables it can't check. Needs a Shopify CLI that runs this package's `check.js`.

check.js

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+#!/usr/bin/env node
+// Shopify CLI runs this, when the package has it, before bundling a JavaScript function:
+//
+//   node check.js --function <function JSON>
+//
+// The function JSON is {handle, directory, entry, schema, targets: [{target, export, inputQuery}]}, where `export` is
+// the JavaScript function the CLI bundles. It exits non-zero, failing the build, when a prepare target returns
+// variables its run query doesn't accept, and warns about variables it can't check.
+const fs = require("node:fs");
+const { parseArgs } = require("node:util");
+const { prepareVariablesProblems } = require("./prepare-variables.js");
+
+try {
+  const { values } = parseArgs({ options: { function: { type: "string" } } });
+  if (!values.function) throw new Error("Usage: check.js --function <function JSON>");
+  const fn = { schema: "schema.graphql", targets: [], ...JSON.parse(fs.readFileSync(values.function, "utf8")) };
+  const name = fn.handle ? `The function extension "${fn.handle}"` : "The function";
+  let problems;
+  try {
+    problems = prepareVariablesProblems(fn);
+  } catch (error) {
+    // A problem in the check itself shouldn't stop the build
+    console.warn(`${name}'s prepare variables couldn't be checked: ${error.message}`);
+    problems = [];
+  }
+  const list = (warning) => problems.filter((problem) => problem.warning === warning).map((problem) => problem.message);
+
+  const warnings = list(true);
+  if (warnings.length > 0) {
+    console.warn(`${name} returns prepare variables that can't be checked against its run query\n${warnings.join("\n")}`);
+  }
+  const errors = list(false);
+  if (errors.length > 0) {
+    throw new Error(`${name} returns prepare variables its run query doesn't accept\n${errors.join("\n")}`);
+  }
+} catch (error) {
+  console.error(error.message);
+  process.exitCode = 1;
+}