[Robustness] nvm_install_binary_extract: replace a broken version dir atomically

A version directory left without a working `bin/node`
- e.g. a partial or interrupted earlier install
- wedged reinstallation:
`nvm install` saw the version as not installed and re-extracted,
but the per-entry `mv` refused to overwrite the leftover non-empty `bin/`,
`lib/`, … subdirectories and left a half-updated tree behind
(and, without `-b`, fell back to a from-source compile)

Remove any pre-existing version directory and move the freshly extracted tree into place with a single rename,
so a version is either fully installed or not present at all.
The removal is safe: it runs only after the tarball has downloaded and extracted successfully into the cache.
Fall back to the previous per-entry move when a single rename cannot cross filesystems.

nvm.sh

@@ -2377,22 +2377,39 @@ nvm_install_binary_extract() {
   if [ "${NVM_OS}" = 'win' ]; then
     VERSION_PATH="${VERSION_PATH}/bin"
     command unzip -q "${TARBALL}" -d "${TMPDIR}" || return 1
-  # For non Windows system (including WSL running on Windows)
-  else
-    nvm_extract_tarball "${NVM_OS}" "${VERSION}" "${TARBALL}" "${TMPDIR}"
-  fi
-
-  command mkdir -p "${VERSION_PATH}" || return 1
-
-  if [ "${NVM_OS}" = 'win' ]; then
+    # Replace any pre-existing (possibly broken or partial) install so the
+    # move below cannot collide with leftover files. Safe here: the archive
+    # has already downloaded and unzipped successfully into TMPDIR.
+    command rm -rf "${VERSION_PATH}"
+    command mkdir -p "${VERSION_PATH}" || return 1
     command mv "${TMPDIR}/"*/* "${VERSION_PATH}/" || return 1
     command chmod +x "${VERSION_PATH}"/node.exe || return 1
     command chmod +x "${VERSION_PATH}"/npm || return 1
     command chmod +x "${VERSION_PATH}"/npx 2>/dev/null
-  else
-    command mv "${TMPDIR}/"* "${VERSION_PATH}" || return 1
+    command rm -rf "${TMPDIR}"
+    return 0
   fi
 
+  # For non-Windows systems (including WSL running on Windows)
+  nvm_extract_tarball "${NVM_OS}" "${VERSION}" "${TARBALL}" "${TMPDIR}" || return 1
+
+  # Install atomically: replace any pre-existing version directory with a
+  # single rename, so a partial or broken tree is never observed as installed.
+  # A leftover directory without a working bin/node otherwise wedges the
+  # install - the per-entry `mv` refuses to overwrite the non-empty bin/, lib/,
+  # ... subdirectories and leaves a half-updated tree behind. Removing it first
+  # is safe: the tarball has already downloaded and extracted into TMPDIR.
+  command rm -rf "${VERSION_PATH}" || return 1
+  command mkdir -p "$(dirname "${VERSION_PATH}")" || return 1
+  if command mv "${TMPDIR}" "${VERSION_PATH}" 2>/dev/null; then
+    return 0
+  fi
+
+  # Fall back to a per-entry move when a single rename is not possible (e.g.
+  # TMPDIR and the versions directory are on different filesystems).
+  command rm -rf "${VERSION_PATH}"
+  command mkdir -p "${VERSION_PATH}" || return 1
+  command mv "${TMPDIR}/"* "${VERSION_PATH}" || return 1
   command rm -rf "${TMPDIR}"
 
   return 0

test/fast/Unit tests/nvm_install_binary_extract self-heals a broken version dir

@@ -0,0 +1,62 @@
+#!/bin/sh
+
+cleanup() {
+  [ -n "${tmp_dir}" ] && [ -d "${tmp_dir}" ] && rm -rf "${tmp_dir}"
+  [ -n "${NVM_DIR}" ] && [ -d "${NVM_DIR}" ] && rm -rf "${NVM_DIR}"
+  unset -f die cleanup nvm_supports_xz
+  unset NVM_DIR tmp_dir version archi node_dir version_path
+}
+
+die() { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+# Use gzip so the test does not depend on xz being installed.
+nvm_supports_xz() { return 1; }
+
+set -ex
+
+type nvm_install_binary_extract > /dev/null 2>&1 || die 'nvm_install_binary_extract is not available'
+
+NVM_DIR="$(mktemp -d)"
+tmp_dir="$(mktemp -d)"
+[ -n "${NVM_DIR}" ] && [ -n "${tmp_dir}" ] || die 'Unable to create temporary folders'
+
+version='v14.15.4'
+archi='linux-x64'
+node_dir="${tmp_dir}/node-${version}-${archi}"
+
+# Build a fake binary tarball: a runnable node, an npm symlink, and a lib tree.
+mkdir -p "${node_dir}/bin" "${node_dir}/lib/node_modules/npm/bin" || die 'setup mkdir failed'
+printf '#!/bin/sh\necho "%s"\n' "${version}" > "${node_dir}/bin/node"
+chmod +x "${node_dir}/bin/node"
+echo 'npm-cli' > "${node_dir}/lib/node_modules/npm/bin/npm-cli.js"
+( cd "${node_dir}/bin" && ln -s ../lib/node_modules/npm/bin/npm-cli.js npm )
+echo 'fresh' > "${node_dir}/lib/FRESHFILE"
+( cd "${tmp_dir}" && tar -czf "${node_dir}.tar.gz" "node-${version}-${archi}" ) || die 'unable to create fake tarball'
+
+# Pre-create a BROKEN, non-empty version directory: npm and a stale lib file
+# are present, but bin/node is missing - exactly the shape that wedges a
+# per-entry move onto existing non-empty subdirectories.
+version_path="${NVM_DIR}/versions/node/${version}"
+mkdir -p "${version_path}/bin" "${version_path}/lib" || die 'unable to stage broken dir'
+echo 'stale-npm' > "${version_path}/bin/npm"
+echo 'stale' > "${version_path}/lib/STALEFILE"
+[ -e "${version_path}/bin/node" ] && die 'precondition: broken dir should have no bin/node'
+
+# Extract over the broken directory.
+nvm_install_binary_extract 'linux' "${version}" "$(expr "${version}" : '.\(.*\)')" "${node_dir}.tar.gz" "${tmp_dir}/files" || die 'nvm_install_binary_extract failed over a broken version dir'
+
+# bin/node is restored and runnable.
+[ -x "${version_path}/bin/node" ] || die 'bin/node was not restored'
+[ "$("${version_path}/bin/node")" = "${version}" ] || die 'restored bin/node has wrong contents'
+
+# The stale file is gone: the whole directory was replaced, not merged into.
+[ ! -e "${version_path}/lib/STALEFILE" ] || die 'stale lib file survived: install was not atomic'
+[ -e "${version_path}/lib/FRESHFILE" ] || die 'fresh lib content missing after install'
+
+# npm resolves to the freshly installed target.
+[ -e "${version_path}/bin/npm" ] || die 'npm missing after install'
+
+cleanup