shell mv: trim the preallocated destination to the bytes copied on the cross-device fallback

src/runtime/shell/builtin/mv.rs

@@ -612,6 +612,16 @@ impl ShellMvBatchedTask {
             let _ = bun_sys::unlinkat(dst_dir, dst);
             return Err(e);
         }
+        // The source can hold fewer bytes than `st_size` said (it shrank, or it
+        // is a sysfs file). The copy advanced the source position by the bytes
+        // it copied; FICLONE does not move it, so fall back to the source size.
+        let copied = match bun_sys::lseek(in_.fd(), 0, libc::SEEK_CUR) {
+            Ok(pos) if pos > 0 => pos,
+            _ => bun_sys::fstat(in_.fd()).map_or(st.st_size as i64, |s| s.st_size as i64),
+        };
+        if copied < st.st_size as i64 {
+            let _ = bun_sys::ftruncate(out.fd(), copied);
+        }
         #[cfg(unix)]
         {
             // `fchown` first: Linux clears S_ISUID/S_ISGID on chown.

test/js/bun/shell/commands/mv.test.ts

@@ -1,6 +1,6 @@
 import { $ } from "bun";
 import { describe, expect, test } from "bun:test";
-import { isPosix } from "harness";
+import { isPosix, tempDir } from "harness";
 import {
   accessSync,
   chmodSync,
@@ -118,6 +118,24 @@ describe("mv", async () => {
       }
     });
 
+    // The copy fallback preallocates the destination to the source's st_size.
+    // A sysfs file reports st_size 4096 but holds a few bytes, so the
+    // destination must shrink to the bytes copied. The unlink of the sysfs
+    // source fails, so mv exits non-zero, but the copy has already happened.
+    const sysfsFile = "/sys/devices/system/cpu/online";
+    const hasSysfs = process.platform === "linux" && existsSync(sysfsFile);
+    test.skipIf(!hasSysfs)("across devices, the destination is the bytes copied, not st_size", async () => {
+      using dst = tempDir("bun-mv-xdev-short-read", {});
+      const content = readFileSync(sysfsFile);
+      expect(content.length).toBeGreaterThan(0);
+      expect(statSync(sysfsFile).size).toBeGreaterThan(content.length);
+      const dstFile = join(String(dst), "online");
+
+      const r = await $`mv ${sysfsFile} ${dstFile}`.quiet();
+      expect(r.exitCode).not.toBe(0);
+      expect(readFileSync(dstFile)).toEqual(content);
+    });
+
     test.skipIf(skip)("file -> directory across devices", async () => {
       const [src, dst] = crossDevicePair("into-dir");
       try {