don't clobber an existing file when building the site

closes #1367
supersedes #1369
supersedes #1373

src/build.ts

@@ -1,10 +1,10 @@
 import {createHash} from "node:crypto";
 import {existsSync} from "node:fs";
-import {access, constants, copyFile, readFile, stat, writeFile} from "node:fs/promises";
+import {access, constants, copyFile, readFile, realpath, stat, writeFile} from "node:fs/promises";
 import {basename, dirname, extname, join} from "node:path/posix";
 import type {Config} from "./config.js";
 import {CliError, isEnoent} from "./error.js";
-import {getClientPath, prepareOutput, visitMarkdownFiles} from "./files.js";
+import {getClientPath, maybeStat, prepareOutput, visitMarkdownFiles} from "./files.js";
 import {getModuleHash} from "./javascript/module.js";
 import {transpileModule} from "./javascript/transpile.js";
 import type {Logger, Writer} from "./logger.js";
@@ -345,13 +345,25 @@ export class FileBuildEffects implements BuildEffects {
   async copyFile(sourcePath: string, outputPath: string): Promise<void> {
     const destination = join(this.outputRoot, outputPath);
     this.logger.log(destination);
+    await noClobber(destination);
     await prepareOutput(destination);
     await copyFile(sourcePath, destination);
   }
   async writeFile(outputPath: string, contents: string | Buffer): Promise<void> {
     const destination = join(this.outputRoot, outputPath);
     this.logger.log(destination);
+    await noClobber(destination);
     await prepareOutput(destination);
     await writeFile(destination, contents);
   }
 }
+
+// Asserts that the destination file is not already present. This can happen if
+// the same file is referenced with inconsistent naming (e.g. upper vs.
+// lowercase), and would result in a broken site.
+async function noClobber(path) {
+  if (await maybeStat(path)) {
+    const real = (await realpath(path)).slice((await realpath(".")).length + 1);
+    throw new Error(`${faint("File name conflict over")} ${real}.`);
+  }
+}