Explain ERESOLVE errors

When peerDependencies conflict, Arborist is now providing details of the
nodes and their reasons for inclusion on the Error object, including
whether or not this resolution error could be overridden using the
--force flag.

Print this data out in a minimal way as a warning if we override an
ERESOLVE forcefully.  When the ERESOLVE causes the install to fail,
print a somewhat longer message, and save a MUCH longer full report to
the cache folder.

lib/utils/error-message.js

@@ -4,11 +4,18 @@ const { format } = require('util')
 const { resolve } = require('path')
 const nameValidator = require('validate-npm-package-name')
 const npmlog = require('npmlog')
+const { report: explainEresolve } = require('./explain-eresolve.js')
 
 module.exports = (er) => {
   const short = []
   const detail = []
   switch (er.code) {
+    case 'ERESOLVE':
+      short.push(['ERESOLVE', er.message])
+      detail.push(['', ''])
+      detail.push(['', explainEresolve(er)])
+      break
+
     case 'ENOLOCK': {
       const cmd = npm.command || ''
       short.push([cmd, 'This command requires an existing lockfile.'])

lib/utils/explain-eresolve.js

@@ -0,0 +1,146 @@
+// this is called when an ERESOLVE error is caught in the error-handler,
+// or when there's a log.warn('eresolve', msg, explanation), to turn it
+// into a human-intelligible explanation of what's wrong and how to fix.
+//
+// TODO: abstract out the explainNode methods into a separate util for
+// use by a future `npm explain <path || spec>` command.
+
+const npm = require('../npm.js')
+const { writeFileSync } = require('fs')
+const { resolve } = require('path')
+
+const chalk = require('chalk')
+const nocolor = {
+  bold: s => s,
+  dim: s => s
+}
+
+// expl is an explanation object that comes from Arborist.  It looks like:
+// {
+//   dep: {
+//     whileInstalling: {
+//       explanation of the thing being installed when we hit the conflict
+//     },
+//     name,
+//     version,
+//     dependents: [
+//       things depending on this node (ie, reason for inclusion)
+//       { name, version, dependents }, ...
+//     ]
+//   }
+//   current: {
+//     explanation of the current node that already was in the tree conflicting
+//   }
+//   peerConflict: {
+//     explanation of the peer dependency that couldn't be added, or null
+//   }
+//   fixWithForce: Boolean - can we use --force to push through this?
+//   type: type of the edge that couldn't be met
+//   isPeer: true if the edge that couldn't be met is a peer dependency
+// }
+// Depth is how far we want to want to descend into the object making a report.
+// The full report (ie, depth=Infinity) is always written to the cache folder
+// at ${cache}/eresolve-report.txt along with full json.
+const explainEresolve = (expl, color, depth) => {
+  const { dep, current, peerConflict } = expl
+
+  const out = []
+  /* istanbul ignore else - should always have this for ERESOLVEs */
+  if (dep.whileInstalling) {
+    out.push('While resolving: ' + printNode(dep.whileInstalling, color))
+  }
+
+  out.push(explainNode('Found:', current, depth, color))
+
+  out.push(explainNode('\nCould not add conflicting dependency:', dep, depth, color))
+
+  if (peerConflict) {
+    const heading = '\nConflicting peer dependency:'
+    const pc = explainNode(heading, peerConflict, depth, color)
+    out.push(pc)
+  }
+
+  return out.join('\n')
+}
+
+const explainNode = (heading, node, depth, color) =>
+  `${heading} ${printNode(node, color)}` +
+  explainDependents(node, depth, color)
+
+const printNode = ({ name, version, location }, color) => {
+  const { bold, dim } = color ? chalk : nocolor
+  return `${bold(name)}@${bold(version)}` +
+    (location ? dim(` at ${location}`) : '')
+}
+
+const explainDependents = ({name, dependents}, depth, color) => {
+  if (!dependents || !dependents.length || depth <= 0)
+    return ''
+
+  const max = Math.ceil(depth / 2)
+  const messages = dependents.slice(0, max)
+    .map(dep => explainDependency(name, dep, depth, color))
+
+  // show just the names of the first 5 deps that overflowed the list
+  if (dependents.length > max) {
+    const names = dependents.slice(max).map(d => d.from.name)
+    const showNames = names.slice(0, 5)
+    if (showNames.length < names.length)
+      showNames.push('...')
+    const show = `(${showNames.join(', ')})`
+    messages.push(`${names.length} more ${show}`)
+  }
+
+  const str = '\nfor: ' + messages.join('\nand: ')
+  return str.split('\n').join('\n  ')
+}
+
+const explainDependency = (name, {type, from, spec}, depth, color) => {
+  const { bold } = color ? chalk : nocolor
+  return `${type} dependency ` +
+    `${bold(name)}@"${bold(spec)}"\nfrom: ` +
+    explainFrom(from, depth, color)
+}
+
+const explainFrom = (from, depth, color) => {
+  if (!from.name && !from.version) {
+    return 'the root project'
+  }
+
+  return printNode(from, color) +
+    explainDependents(from, depth - 1, color)
+}
+
+// generate a full verbose report and tell the user how to fix it
+const report = (expl, depth = 4) => {
+  const fullReport = resolve(npm.cache, 'eresolve-report.txt')
+
+  const orForce = expl.fixWithForce ? ' or --force' : ''
+  const fix =`Fix the upstream dependency conflict, or retry
+this command with --legacy-peer-deps${orForce}
+to accept an incorrect (and potentially broken) dependency resolution.`
+
+  writeFileSync(fullReport, `# npm resolution error report
+
+${new Date().toISOString()}
+
+${explainEresolve(expl, false, Infinity)}
+
+${fix}
+
+Raw JSON explanation object:
+
+${JSON.stringify(expl, null, 2)}
+`, 'utf8')
+
+  return explainEresolve(expl, npm.color, depth) +
+    `\n\n${fix}\n\nSee ${fullReport} for a full report.`
+}
+
+// the terser explain method for the warning when using --force
+const explain = (expl, depth = 2) => explainEresolve(expl, npm.color, depth)
+
+module.exports = {
+  explain,
+  report
+}

lib/utils/setup-log.js

@@ -1,10 +1,27 @@
 // module to set the appropriate log settings based on configs
 // returns a boolean to say whether we should enable color on
 // stdout or not.
+//
+// Also (and this is a really inexcusable kludge), we patch the
+// log.warn() method so that when we see a peerDep override
+// explanation from Arborist, we can replace the object with a
+// highly abbreviated explanation of what's being overridden.
 const log = require('npmlog')
+const { explain } = require('./explain-eresolve.js')
+
 module.exports = (config) => {
   const color = config.get('color')
 
+  const { warn } = log
+
+  log.warn = (heading, ...args) => {
+    if (heading === 'ERESOLVE' && args[1] && typeof args[1] === 'object') {
+      warn(heading, args[0])
+      return warn('', explain(args[1]))
+    }
+    return warn(heading, ...args)
+  }
+
   if (config.get('timing') && config.get('loglevel') === 'notice') {
     log.level = 'timing'
   } else {

tap-snapshots/test-lib-utils-error-message.js-TAP.test.js

@@ -782,6 +782,27 @@ Object {
 }
 `
 
+exports[`test/lib/utils/error-message.js TAP explain ERESOLVE errors > must match snapshot 1`] = `
+Object {
+  "detail": Array [
+    Array [
+      "",
+      "",
+    ],
+    Array [
+      "",
+      "explanation",
+    ],
+  ],
+  "summary": Array [
+    Array [
+      "ERESOLVE",
+      "could not resolve",
+    ],
+  ],
+}
+`
+
 exports[`test/lib/utils/error-message.js TAP just simple messages > must match snapshot 1`] = `
 Object {
   "detail": Array [],