[Fix] nvm install: migrate packages/set alias when target is already installed

When `nvm install <target>` found that `<target>` was already installed,
the post-`nvm use` steps
(`--reinstall-packages-from`, default packages, and `--alias`/`--default`)
were gated on `[ $EXIT_CODE -ne 0 ]`.
Since that branch is only entered when `nvm use` succeeded (EXIT_CODE == 0),
those conditions were always false, so the steps were silently skipped.

The fresh-install branch correctly uses `-eq 0`;
mirror that here so `--reinstall-packages-from` actually migrates global packages
(and the alias/default get set) when the target version already exists.

Includes a regression test covering the already-installed path.

Fixes #3858

nvm.sh

@@ -3736,10 +3736,10 @@ nvm() {
             nvm install-latest-npm
             EXIT_CODE=$?
           fi
-          if [ $EXIT_CODE -ne 0 ] && [ -z "${SKIP_DEFAULT_PACKAGES-}" ]; then
+          if [ $EXIT_CODE -eq 0 ] && [ -z "${SKIP_DEFAULT_PACKAGES-}" ]; then
             nvm_install_default_packages
           fi
-          if [ $EXIT_CODE -ne 0 ] && [ -n "${REINSTALL_PACKAGES_FROM-}" ] && [ "_${REINSTALL_PACKAGES_FROM}" != "_N/A" ]; then
+          if [ $EXIT_CODE -eq 0 ] && [ -n "${REINSTALL_PACKAGES_FROM-}" ] && [ "_${REINSTALL_PACKAGES_FROM}" != "_N/A" ]; then
             nvm reinstall-packages "${REINSTALL_PACKAGES_FROM}"
             EXIT_CODE=$?
           fi
@@ -3757,7 +3757,7 @@ nvm() {
           EXIT_CODE=$?
         fi
 
-        if [ $EXIT_CODE -ne 0 ] && [ -n "${ALIAS-}" ]; then
+        if [ $EXIT_CODE -eq 0 ] && [ -n "${ALIAS-}" ]; then
           nvm alias "${ALIAS}" "${provided_version}"
           EXIT_CODE=$?
         fi

test/installation_node/install while reinstalling packages

@@ -1,6 +1,27 @@
 #!/bin/sh
 
-die () { echo "$@" ; exit 1; }
+# State touched by the regression section below; cleanup restores it even on failure.
+DEFAULT_PACKAGES_FILE="${NVM_DIR}/default-packages"
+DEFAULT_PACKAGES_BACKUP=""
+DEFAULT_PACKAGES_WROTE=""
+TEST_ALIAS=""
+
+cleanup () {
+  if [ -n "${DEFAULT_PACKAGES_WROTE}" ]; then
+    rm -f "${DEFAULT_PACKAGES_FILE}"
+    DEFAULT_PACKAGES_WROTE=""
+  fi
+  if [ -n "${DEFAULT_PACKAGES_BACKUP}" ] && [ -f "${DEFAULT_PACKAGES_BACKUP}" ]; then
+    mv "${DEFAULT_PACKAGES_BACKUP}" "${DEFAULT_PACKAGES_FILE}"
+    DEFAULT_PACKAGES_BACKUP=""
+  fi
+  if [ -n "${TEST_ALIAS}" ]; then
+    nvm unalias "${TEST_ALIAS}" > /dev/null 2>&1 || true
+    TEST_ALIAS=""
+  fi
+}
+
+die () { echo "$@" ; cleanup ; exit 1; }
 
 \. ../../nvm.sh
 
@@ -44,3 +65,54 @@ nvm use --lts=argon
 node --version | grep v4.9.1 > /dev/null || die "nvm ls --lts=argon didn't use v4.9.1"
 
 npm list --global | grep object-is > /dev/null || die "object-is isn't installed"
+
+
+# Regression: `--reinstall-packages-from` must still migrate packages when the
+# target version is *already installed*. See https://github.com/nvm-sh/nvm/issues/3858
+
+# install a fresh global package on the source (v9.7.0) only
+nvm use 9.7.0
+npm install -g is-nan@1.0.0 || die "npm install -g is-nan failed"
+npm list --global | grep is-nan > /dev/null || die "is-nan isn't installed on v9.7.0"
+
+# precondition: the already-installed target (v9.10.0) must not have it yet
+nvm use 9.10.0
+npm list --global | grep is-nan > /dev/null && die "is-nan should not be installed on v9.10.0 before reinstall"
+
+# target is already installed, so this should report so AND still migrate packages
+OUTPUT="$(nvm install --reinstall-packages-from=9.7.0 9.10.0 2>&1)" || die "nvm install --reinstall-packages-from=9.7.0 9.10.0 failed: ${OUTPUT}"
+nvm_echo "${OUTPUT}" | grep "is already installed" > /dev/null || die "expected 'already installed' message, got: ${OUTPUT}"
+
+nvm use 9.10.0
+npm list --global | grep is-nan > /dev/null || die "is-nan was not migrated to already-installed v9.10.0"
+
+
+# ...the same is true for default packages: they must install on the already-installed path too.
+
+# back up any real default-packages file, then point it at a package the target lacks
+if [ -f "${DEFAULT_PACKAGES_FILE}" ]; then
+  DEFAULT_PACKAGES_BACKUP="${DEFAULT_PACKAGES_FILE}.3858.bak"
+  mv "${DEFAULT_PACKAGES_FILE}" "${DEFAULT_PACKAGES_BACKUP}" || die "could not back up ${DEFAULT_PACKAGES_FILE}"
+fi
+DEFAULT_PACKAGES_WROTE=1
+nvm_echo 'object-inspect@1.0.2' > "${DEFAULT_PACKAGES_FILE}"
+
+# precondition: the already-installed target must not have the default package yet
+nvm use 9.10.0
+npm list --global | grep object-inspect > /dev/null && die "object-inspect should not be installed on v9.10.0 before default-packages test"
+
+nvm install 9.10.0 > /dev/null 2>&1 || die "nvm install 9.10.0 (already installed, default-packages) failed"
+nvm use 9.10.0
+npm list --global | grep object-inspect > /dev/null || die "default packages were not installed on already-installed v9.10.0"
+
+# restore the default-packages file before the next assertion
+cleanup
+
+
+# ...and `--alias` must be set on the already-installed path too.
+TEST_ALIAS='nvm-3858-already-installed'
+nvm unalias "${TEST_ALIAS}" > /dev/null 2>&1 || true # ensure a clean precondition
+nvm install --alias="${TEST_ALIAS}" 9.10.0 > /dev/null 2>&1 || die "nvm install --alias=${TEST_ALIAS} 9.10.0 (already installed) failed"
+TERM=dumb nvm alias "${TEST_ALIAS}" | grep 'v9.10.0' > /dev/null || die "--alias was not set on already-installed v9.10.0"
+
+cleanup