Avoid partial updates from applying changes to packages which are not locked with an acceptable stability

src/Composer/DependencyResolver/PoolBuilder.php

@@ -16,7 +16,9 @@ use Composer\Package\AliasPackage;
 use Composer\Package\BasePackage;
 use Composer\Package\Package;
 use Composer\Package\PackageInterface;
+use Composer\Package\Version\StabilityFilter;
 use Composer\Repository\PlatformRepository;
+use Composer\Repository\RootPackageRepository;
 use Composer\Semver\Constraint\Constraint;
 use Composer\Semver\Constraint\MultiConstraint;
 
@@ -57,7 +59,12 @@ class PoolBuilder
             $this->nameConstraints[$package->getName()] = null;
             $this->loadedNames[$package->getName()] = true;
             unset($loadNames[$package->getName()]);
-            $loadNames += $this->loadPackage($request, $package);
+            if (
+                $package->getRepository() instanceof RootPackageRepository
+                || StabilityFilter::isPackageAcceptable($this->acceptableStabilities, $this->stabilityFlags, $package->getNames(), $package->getStability())
+            ) {
+                $loadNames += $this->loadPackage($request, $package);
+            }
         }
 
         foreach ($request->getJobs() as $job) {

src/Composer/DependencyResolver/RuleSetGenerator.php

@@ -290,8 +290,9 @@ class RuleSetGenerator
         $unlockableMap = $request->getUnlockableMap();
 
         foreach ($request->getFixedPackages() as $package) {
+            // fixed package was not added to the pool which must mean it did not pass the stability requirements
             if ($package->id == -1) {
-                throw new \RuntimeException("Fixed package ".$package->getName()." ".$package->getVersion().($package instanceof AliasPackage ? " (alias)" : "")." was not added to solver pool.");
+                continue;
             }
 
             $this->addRulesForPackage($package, $ignorePlatformReqs);

src/Composer/Installer.php

@@ -380,12 +380,10 @@ class Installer
         }
 
         // if the updateWhitelist is enabled, packages not in it are also fixed
-        // to the version specified in the lock, except if their stability is not
-        // acceptable anymore, to make sure that they get updated/downgraded to
-        // a working version
+        // to the version specified in the lock
         if ($this->updateWhitelist && $lockedRepository) {
             foreach ($lockedRepository->getPackages() as $lockedPackage) {
-                if (!$this->isUpdateable($lockedPackage) && $repositorySet->isPackageAcceptable($lockedPackage->getNames(), $lockedPackage->getStability())) {
+                if (!$this->isUpdateable($lockedPackage)) {
                     // TODO add reason for fix?
                     $request->fixPackage($lockedPackage);
                 }