Enable copying symlinks when using remote docker.

This handles the reads the link dst from the symlink when using
temporary directories, and creates a softlink to the path (keeping
relative and absolute symlinks valid) in the temporary directory, which
is then copied with the same permissions over the dst filesystem. This
should not keep the same metadata, but since we fingerprint source
files, this should not matter.

src/docker/remote.rs

@@ -171,14 +171,16 @@ fn copy_volume_files_nocache(
     container: &str,
     src: &Path,
     dst: &Path,
+    copy_symlinks: bool,
     msg_info: MessageInfo,
 ) -> Result<ExitStatus> {
     // avoid any cached directories when copying
     // see https://bford.info/cachedir/
     // SAFETY: safe, single-threaded execution.
     let tempdir = unsafe { temp::TempDir::new()? };
     let temppath = tempdir.path();
-    copy_dir(src, temppath, 0, |e, _| is_cachedir(e))?;
+    let had_symlinks = copy_dir(src, temppath, copy_symlinks, 0, |e, _| is_cachedir(e))?;
+    warn_symlinks(had_symlinks, msg_info)?;
     copy_volume_files(engine, container, temppath, dst, msg_info)
 }
 
@@ -234,10 +236,18 @@ pub fn copy_volume_container_cargo(
 }
 
 // recursively copy a directory into another
-fn copy_dir<Skip>(src: &Path, dst: &Path, depth: u32, skip: Skip) -> Result<()>
+fn copy_dir<Skip>(
+    src: &Path,
+    dst: &Path,
+    copy_symlinks: bool,
+    depth: u32,
+    skip: Skip,
+) -> Result<bool>
 where
     Skip: Copy + Fn(&fs::DirEntry, u32) -> bool,
 {
+    let mut had_symlinks = false;
+
     for entry in fs::read_dir(src)? {
         let file = entry?;
         if skip(&file, depth) {
@@ -248,13 +258,47 @@ where
         let dst_path = dst.join(file.file_name());
         if file.file_type()?.is_file() {
             fs::copy(&src_path, &dst_path)?;
-        } else {
+        } else if file.file_type()?.is_dir() {
             fs::create_dir(&dst_path).ok();
-            copy_dir(&src_path, &dst_path, depth + 1, skip)?;
+            had_symlinks = copy_dir(&src_path, &dst_path, copy_symlinks, depth + 1, skip)?;
+        } else if copy_symlinks {
+            had_symlinks = true;
+            let link_dst = fs::read_link(src_path)?;
+
+            #[cfg(target_family = "unix")]
+            {
+                std::os::unix::fs::symlink(link_dst, dst_path)?;
+            }
+
+            #[cfg(target_family = "windows")]
+            {
+                let link_dst_absolute = if link_dst.is_absolute() {
+                    link_dst
+                } else {
+                    // we cannot fail even if the linked to path does not exist.
+                    src.join(link_dst)
+                };
+                if link_dst_absolute.is_dir() {
+                    std::os::windows::fs::symlink_dir(link_dst, dst_path)?;
+                } else {
+                    // symlink_file handles everything that isn't a directory
+                    std::os::windows::fs::symlink_file(link_dst, dst_path)?;
+                }
+            }
+        } else {
+            had_symlinks = true;
         }
     }
 
-    Ok(())
+    Ok(had_symlinks)
+}
+
+fn warn_symlinks(had_symlinks: bool, msg_info: MessageInfo) -> Result<()> {
+    if had_symlinks {
+        shell::warn("copied directory contained symlinks. if the volume the link points to was not mounted, the remote build may fail", msg_info)
+    } else {
+        Ok(())
+    }
 }
 
 // copy over files needed for all targets in the toolchain that should never change
@@ -284,20 +328,25 @@ fn copy_volume_container_rust_base(
     let tempdir = unsafe { temp::TempDir::new()? };
     let temppath = tempdir.path();
     fs::create_dir_all(&temppath.join(&rustlib))?;
-    copy_dir(&sysroot.join("lib"), &temppath.join("lib"), 0, |e, d| {
-        d == 0 && e.file_name() == "rustlib"
-    })?;
+    let mut had_symlinks = copy_dir(
+        &sysroot.join("lib"),
+        &temppath.join("lib"),
+        true,
+        0,
+        |e, d| d == 0 && e.file_name() == "rustlib",
+    )?;
 
     // next, copy the src/etc directories inside rustlib
-    copy_dir(
+    had_symlinks |= copy_dir(
         &sysroot.join(&rustlib),
         &temppath.join(&rustlib),
+        true,
         0,
         |e, d| d == 0 && !(e.file_name() == "src" || e.file_name() == "etc"),
     )?;
     copy_volume_files(engine, container, &temppath.join("lib"), &dst, msg_info)?;
 
-    Ok(())
+    warn_symlinks(had_symlinks, msg_info)
 }
 
 fn copy_volume_container_rust_manifest(
@@ -316,15 +365,16 @@ fn copy_volume_container_rust_manifest(
     let tempdir = unsafe { temp::TempDir::new()? };
     let temppath = tempdir.path();
     fs::create_dir_all(&temppath.join(&rustlib))?;
-    copy_dir(
+    let had_symlinks = copy_dir(
         &sysroot.join(&rustlib),
         &temppath.join(&rustlib),
+        true,
         0,
         |e, d| d != 0 || e.file_type().map(|t| !t.is_file()).unwrap_or(true),
     )?;
     copy_volume_files(engine, container, &temppath.join("lib"), &dst, msg_info)?;
 
-    Ok(())
+    warn_symlinks(had_symlinks, msg_info)
 }
 
 // copy over the toolchain for a specific triple
@@ -570,7 +620,7 @@ fn copy_volume_container_project(
         if copy_cache {
             copy_volume_files(engine, container, src, dst, msg_info)
         } else {
-            copy_volume_files_nocache(engine, container, src, dst, msg_info)
+            copy_volume_files_nocache(engine, container, src, dst, true, msg_info)
         }
     };
     match volume {
@@ -809,7 +859,7 @@ pub(crate) fn run(
         if copy_cache {
             copy_volume_files(engine, &container, src, dst, msg_info)
         } else {
-            copy_volume_files_nocache(engine, &container, src, dst, msg_info)
+            copy_volume_files_nocache(engine, &container, src, dst, true, msg_info)
         }
     };
     let mount_prefix_path = mount_prefix.as_ref();