[Perf] nvm use: skip re-resolving an exact, installed version

When `nvm use` is given an exact `vX.Y.Z` that is already installed,
and no alias with that exact name exists,
use it directly instead of re-resolving it through
`nvm_match_version` and `nvm_ensure_version_installed`,
both of which call `nvm_version`.

`nvm_auto` takes this path on every shell startup with an already-resolved version,
so this removes ~95ms from each new shell
(~330ms -> ~235ms in a clean environment on macOS/zsh).

Fixes #3930

nvm.sh

@@ -212,6 +212,23 @@ nvm_has_system_iojs() {
   [ "$(nvm deactivate >/dev/null 2>&1 && command -v iojs)" != '' ]
 }
 
+nvm_is_installed_exact_version() {
+  case "${1-}" in
+    v*) ;;
+    *) return 1 ;;
+  esac
+  case "${1#v}" in
+    *[!0-9.]* | .* | *. | *..* | *.*.*.*) return 1 ;;
+    *.*.*) ;;
+    *) return 1 ;;
+  esac
+  # an alias with this exact name takes precedence, as it does in `nvm_version`
+  if [ -e "$(nvm_alias_path)/${1}" ]; then
+    return 1
+  fi
+  nvm_is_version_installed "${1}"
+}
+
 nvm_is_version_installed() {
   if [ -z "${1-}" ]; then
     return 1
@@ -4536,6 +4553,8 @@ nvm() {
       IS_VERSION_FROM_NVMRC=0
       local NVM_WRITE_TO_NVMRC
       NVM_WRITE_TO_NVMRC=0
+      local NVM_USE_EXACT_INSTALLED
+      NVM_USE_EXACT_INSTALLED=0
 
       while [ $# -ne 0 ]; do
         case "$1" in
@@ -4576,6 +4595,9 @@ nvm() {
           nvm_err 'Please see `nvm --help` or https://github.com/nvm-sh/nvm#nvmrc for more information.'
           return 127
         fi
+      elif nvm_is_installed_exact_version "${PROVIDED_VERSION}"; then
+        VERSION="${PROVIDED_VERSION}"
+        NVM_USE_EXACT_INSTALLED=1
       else
         VERSION="$(nvm_match_version "${PROVIDED_VERSION}")"
       fi
@@ -4617,9 +4639,8 @@ nvm() {
           nvm_ensure_version_installed "${PROVIDED_VERSION}" "${IS_VERSION_FROM_NVMRC}"
         fi
         return 3
-      # This nvm_ensure_version_installed call can be a performance bottleneck
-      # on shell startup. Perhaps we can optimize it away or make it faster.
-      elif ! nvm_ensure_version_installed "${VERSION}" "${IS_VERSION_FROM_NVMRC}"; then
+      # an exact, already-installed version needs no further resolution or checks
+      elif [ "${NVM_USE_EXACT_INSTALLED}" -ne 1 ] && ! nvm_ensure_version_installed "${VERSION}" "${IS_VERSION_FROM_NVMRC}"; then
         return $?
       fi
 
@@ -5241,7 +5262,7 @@ nvm() {
         nvm_echo nvm_err nvm_grep nvm_cd \
         nvm_die_on_prefix nvm_get_make_jobs nvm_get_minor_version \
         nvm_has_solaris_binary nvm_is_merged_node_version \
-        nvm_is_natural_num nvm_is_version_installed nvm_validate_install \
+        nvm_is_natural_num nvm_is_installed_exact_version nvm_is_version_installed nvm_validate_install \
         nvm_install_lock_name nvm_acquire_install_lock nvm_release_install_lock \
         nvm_list_aliases nvm_make_alias nvm_print_alias_file nvm_print_alias_path \
         nvm_print_default_alias nvm_print_formatted_alias nvm_resolve_local_alias \

test/fast/Unit tests/nvm use skips re-resolving an exact installed version

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+cleanup () {
+  rm -rf "$NVM_DIR"
+  unset -f die cleanup nvm_version
+}
+die () { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+NVM_DIR="$(mktemp -d)"
+NODE_BIN="$NVM_DIR/versions/node/v12.0.0/bin"
+mkdir -p "$NODE_BIN"
+printf '#!/bin/sh\necho v12.0.0\n' > "$NODE_BIN/node"
+chmod +x "$NODE_BIN/node"
+
+# the slow path resolves through nvm_version; make it fail so only the fast path can succeed
+nvm_version() {
+  return 42
+}
+
+nvm use --silent v12.0.0 || die 'nvm use with an exact, installed version should not need nvm_version'
+case ":${PATH}:" in
+  *":${NODE_BIN}:"*) ;;
+  *) die "nvm use did not put ${NODE_BIN} on PATH; got ${PATH}" ;;
+esac
+
+! nvm use --silent v12 2>/dev/null || die 'nvm use with a partial version should still resolve it through nvm_version'
+
+cleanup

test/fast/Unit tests/nvm_is_installed_exact_version

@@ -0,0 +1,58 @@
+#!/bin/sh
+
+cleanup () {
+  rm -rf "$NVM_DIR"
+  unset -f die cleanup
+  unset NVM_DIR NODE_PATH VERSION
+}
+die () { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+set -ex
+
+NVM_DIR=$(mktemp -d)
+NODE_PATH="$NVM_DIR/versions/node"
+mkdir -p "$NODE_PATH"
+if [ -z "$NODE_PATH" ]; then
+  die 'Unable to create temporary folder'
+fi
+
+# nvm_is_installed_exact_version is available
+type nvm_is_installed_exact_version > /dev/null 2>&1 || die 'nvm_is_installed_exact_version is not available'
+
+# nvm_is_installed_exact_version with no parameter fails
+! nvm_is_installed_exact_version || die 'nvm_is_installed_exact_version without parameter should fail'
+
+# an exact version that is not installed fails
+! nvm_is_installed_exact_version 'v12.0.0' || die 'v12.0.0 is not installed and should fail'
+
+# an exact version whose node binary is not executable fails
+mkdir -p "$NODE_PATH/v12.0.0/bin"
+touch "$NODE_PATH/v12.0.0/bin/node"
+! nvm_is_installed_exact_version 'v12.0.0' || die 'v12.0.0 with a non-executable node should fail'
+
+# an exact, installed version succeeds
+chmod +x "$NODE_PATH/v12.0.0/bin/node"
+nvm_is_installed_exact_version 'v12.0.0' || die 'v12.0.0 is installed and should succeed'
+
+# something shaped almost like vX.Y.Z fails, even when a directory by that name is installed
+for VERSION in 'v12.0.0.0' 'v12.0.0.' 'v12..0' 'v.12.0' 'v12.0'; do
+  mkdir -p "$NODE_PATH/$VERSION/bin"
+  touch "$NODE_PATH/$VERSION/bin/node"
+  chmod +x "$NODE_PATH/$VERSION/bin/node"
+  ! nvm_is_installed_exact_version "$VERSION" || die "$VERSION is not an exact version and should fail, even when installed"
+done
+
+# anything that is not an exact vX.Y.Z fails, even if it would resolve to an installed version
+for VERSION in '12.0.0' '12' 'v12' 'v12.0' 'v12.0.0-rc.1' 'node' 'stable' 'lts/*' 'iojs-v1.0.0' 'system' 'default'; do
+  ! nvm_is_installed_exact_version "$VERSION" || die "$VERSION is not an exact version and should fail"
+done
+
+# an alias with the exact same name takes precedence, so the fast path must not apply
+mkdir -p "$(nvm_alias_path)"
+echo 'v12.0.0' > "$(nvm_alias_path)/v12.0.0"
+! nvm_is_installed_exact_version 'v12.0.0' || die 'an alias named v12.0.0 should take precedence'
+
+cleanup