[New] nvm install: add NVM_NO_SOURCE_FALLBACK to disable the source fallback

By default a failed binary download falls back to compiling node from source,
which is slow and pointless on platforms that always have prebuilt binaries.
`-b` already disables the fallback per-invocation;
`NVM_NO_SOURCE_FALLBACK=1` makes it the default for every `nvm install`,
so CI images and dev setups need not thread `-b` through every callsite.

It behaves exactly like `-b`
- aborting with a non-zero exit on a failed binary download
- and is mutually exclusive with `-s`, for the same reason `-b` is.

README.md

@@ -872,6 +872,10 @@ nvm exposes the following environment variables:
 
 Additionally, nvm modifies `PATH`, and, if present, `MANPATH` and `NODE_PATH` when changing versions.
 
+The following environment variables can be set to configure `nvm install`:
+
+- `NVM_NO_SOURCE_FALLBACK` - when `1`, a failed binary download aborts instead of silently falling back to a (much slower) from-source compile; the persistent equivalent of the `-b` flag, and mutually exclusive with `-s`.
+
 
 ## Bash Completion
 

nvm.sh

@@ -3362,6 +3362,7 @@ nvm() {
         nvm_echo '   The following optional arguments, if provided, must appear directly after `nvm install`:'
         nvm_echo '    -s                                        Skip binary download, install from source only.'
         nvm_echo '    -b                                        Skip source download, install from binary only.'
+        nvm_echo '                                              (set NVM_NO_SOURCE_FALLBACK=1 to make this the default for every install)'
         nvm_echo '    --reinstall-packages-from=<version>       When installing, reinstall packages installed in <node|iojs|node version number>'
         nvm_echo '    --lts                                     When installing, only select from LTS (long-term support) versions'
         nvm_echo '    --lts=<LTS name>                          When installing, only select from versions for a specific LTS line'
@@ -3710,6 +3711,17 @@ nvm() {
         esac
       done
 
+      # NVM_NO_SOURCE_FALLBACK=1 makes `nvm install` behave as if `-b` were always passed:
+      # a failed binary download aborts instead of silently falling back to a from-source compile.
+      # It is a persistent policy so callers need not thread `-b` through every invocation.
+      if [ "${NVM_NO_SOURCE_FALLBACK-}" = '1' ] && [ $nosource -ne 1 ]; then
+        if [ $nobinary -eq 1 ]; then
+          nvm_err '-s cannot be combined with NVM_NO_SOURCE_FALLBACK=1 since that would skip install from both binary and source'
+          return 6
+        fi
+        nosource=1
+      fi
+
       if [ "${NVM_OFFLINE}" != 1 ] && ! nvm_has_executable "curl" && ! nvm_has_executable "wget"; then
         nvm_err 'nvm needs curl or wget to proceed.'
         return 1

test/fast/Unit tests/nvm install NVM_NO_SOURCE_FALLBACK

@@ -0,0 +1,50 @@
+#!/bin/sh
+
+cleanup() {
+  [ -n "${NVM_DIR}" ] && [ -d "${NVM_DIR}" ] && rm -rf "${NVM_DIR}"
+  unset -f die cleanup nvm_remote_version nvm_has_executable nvm_binary_available \
+    nvm_install_binary nvm_install_source nvm_get_make_jobs
+  unset NVM_DIR version SOURCE_MARKER OUTPUT EXIT_CODE
+}
+
+die() { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+unset npm_config_prefix NPM_CONFIG_PREFIX
+
+NVM_DIR="$(mktemp -d)"
+[ -n "${NVM_DIR}" ] || die 'unable to create temp NVM_DIR'
+version='v20.0.0'
+SOURCE_MARKER="${NVM_DIR}/source-was-attempted"
+
+# Stay offline; make the binary download always fail and record any attempt to
+# compile from source.
+nvm_remote_version() { nvm_echo "${version}"; }
+nvm_has_executable() { return 0; }
+nvm_binary_available() { return 0; }
+nvm_get_make_jobs() { NVM_MAKE_JOBS=1; }
+nvm_install_binary() { return 1; }
+nvm_install_source() { command touch "${SOURCE_MARKER}"; return 1; }
+
+# Default: a failed binary download falls back to a source compile.
+rm -f "${SOURCE_MARKER}"
+nvm install "${version}" > /dev/null 2>&1
+[ -f "${SOURCE_MARKER}" ] || die 'default behaviour should fall back to source when the binary fails'
+
+# NVM_NO_SOURCE_FALLBACK=1: a failed binary download must NOT compile from source.
+rm -f "${SOURCE_MARKER}"
+NVM_NO_SOURCE_FALLBACK=1 nvm install "${version}" > /dev/null 2>&1; EXIT_CODE=$?
+[ ! -f "${SOURCE_MARKER}" ] || die 'NVM_NO_SOURCE_FALLBACK=1 should not fall back to source'
+[ "${EXIT_CODE}" != "0" ] || die 'a failed binary install with NVM_NO_SOURCE_FALLBACK=1 should be non-zero'
+
+# NVM_NO_SOURCE_FALLBACK=1 combined with -s would skip both methods: a conflict.
+OUTPUT="$(NVM_NO_SOURCE_FALLBACK=1 nvm install -s "${version}" 2>&1)"; EXIT_CODE=$?
+[ "${EXIT_CODE}" = "6" ] || die "expected exit code 6 for -s with NVM_NO_SOURCE_FALLBACK=1, got ${EXIT_CODE}"
+case "${OUTPUT}" in
+  *'skip install from both binary and source'*) ;;
+  *) die "expected a conflict message, got >${OUTPUT}<" ;;
+esac
+
+cleanup