Abort setup on prefix-init failure, flag broken Proton installs

When stepProtonInit failed, the runner kept marching through every
downstream step against a half-initialized prefix, producing a cascade
of misleading "wine client error: version mismatch" failures that
masked the real problem. Seen in the wild with a GE-Proton10-34 install
that was missing concrt140.dll from its bundled default_pfx template:
setup_prefix crashes with FileNotFoundError, then seven unrelated steps
are reported as broken.

Break out of the step loop as soon as proton_init fails, and inspect the
captured Proton output for the "FileNotFoundError" + "default_pfx" hint
so the error message points at the broken Proton distribution instead
of a generic "exit code 1". runProcess gains an optional QByteArray*
out param so the caller can pattern-match on the merged output without
changing any existing call sites.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

2 files changed, 51 insertions, 9 deletions

diff --git a/src/src/prefixsetuprunner.cpp b/src/src/prefixsetuprunner.cpp
index 74d59bc..bc1b9ce 100644
--- a/src/src/prefixsetuprunner.cpp
+++ b/src/src/prefixsetuprunner.cpp
@@ -484,8 +484,20 @@ void PrefixSetupRunner::start()
     if (m_steps[i].status == SetupStep::Succeeded)
       continue;
 
-    allOk = runStep(i) && allOk;
+    const bool stepOk = runStep(i);
+    allOk             = stepOk && allOk;
     emit progressChanged(static_cast<float>(i + 1) / total);
+
+    // The prefix-init step is a hard prerequisite for everything that
+    // follows. If it fails (e.g. broken Proton install), running downstream
+    // steps just produces a cascade of misleading "version mismatch" errors
+    // against a half-initialized prefix. Surface the real error and stop.
+    if (!stepOk && m_steps[i].id == "proton_init") {
+      emit logMessage(
+          "Prefix initialization failed — skipping remaining setup steps. "
+          "Fix the Proton installation and retry.");
+      break;
+    }
   }
 
   emit finished(allOk);
@@ -625,7 +637,8 @@ QProcess* PrefixSetupRunner::buildWrappedProcess(
 int PrefixSetupRunner::runProcess(const QString& exe,
                                   const QStringList& args,
                                   const QMap<QString, QString>& extraEnv,
-                                  int timeoutMs)
+                                  int timeoutMs,
+                                  QByteArray* captured)
 {
   QProcess* proc = buildWrappedProcess(exe, extraEnv);
 
@@ -643,8 +656,10 @@ int PrefixSetupRunner::runProcess(const QString& exe,
 
     if (proc->canReadLine() || proc->bytesAvailable() > 0) {
       const QByteArray data = proc->readAll();
-      if (!data.isEmpty())
+      if (!data.isEmpty()) {
+        if (captured) captured->append(data);
         emitFilteredOutput(this, data);
+      }
     }
 
     if (isCancelled()) {
@@ -656,8 +671,10 @@ int PrefixSetupRunner::runProcess(const QString& exe,
   }
 
   const QByteArray remaining = proc->readAll();
-  if (!remaining.isEmpty())
+  if (!remaining.isEmpty()) {
+    if (captured) captured->append(remaining);
     emitFilteredOutput(this, remaining);
+  }
 
   const int exitCode = proc->exitCode();
   proc->deleteLater();
@@ -785,12 +802,33 @@ bool PrefixSetupRunner::stepProtonInit()
 
   emit logMessage("Initializing Wine prefix with Proton...");
 
+  QByteArray protonOutput;
   const int rc = runProcess(protonScript,
                             {"run", "wineboot", "-u"},
-                            env);
+                            env, -1, &protonOutput);
   if (rc != 0) {
-    m_steps.last().errorMessage =
-        QStringLiteral("proton wineboot failed (exit code %1)").arg(rc);
+    // Detect a broken Proton install: setup_prefix copies DLLs from Proton's
+    // bundled default_pfx template, so a FileNotFoundError there means the
+    // Proton distribution itself is missing files — not a Fluorine issue.
+    const QByteArray out = protonOutput;
+    if (out.contains("FileNotFoundError") &&
+        out.contains("default_pfx/drive_c")) {
+      int start = out.indexOf("default_pfx/drive_c");
+      int end   = out.indexOf('\'', start);
+      if (end < 0) end = out.indexOf('"', start);
+      const QString missing =
+          (end > start) ? QString::fromUtf8(out.mid(start, end - start))
+                        : QStringLiteral("default_pfx/drive_c/...");
+      m_steps.last().errorMessage =
+          QStringLiteral(
+              "Proton install is incomplete: missing '%1'. "
+              "Reinstall or verify your Proton build (e.g. GE-Proton) — "
+              "this is not a Fluorine bug.")
+              .arg(missing);
+    } else {
+      m_steps.last().errorMessage =
+          QStringLiteral("proton wineboot failed (exit code %1)").arg(rc);
+    }
     return false;
   }
 
diff --git a/src/src/prefixsetuprunner.h b/src/src/prefixsetuprunner.h
index ed8f12c..5ac71c8 100644
--- a/src/src/prefixsetuprunner.h
+++ b/src/src/prefixsetuprunner.h
@@ -2,6 +2,7 @@
 #define PREFIXSETUPRUNNER_H
 
 #include <QAtomicInt>
+#include <QByteArray>
 #include <QObject>
 #include <QProcess>
 #include <QString>
@@ -63,11 +64,14 @@ private:
   bool isCancelled() const { return m_cancelled.loadAcquire() != 0; }
 
   /// Run an external process with SLR wrapping and log its output.
-  /// Returns exit code (0 = success).
+  /// Returns exit code (0 = success).  If captured is non-null, a copy of
+  /// the merged stdout/stderr output is appended to it for post-mortem
+  /// pattern matching.
   int runProcess(const QString& exe,
                  const QStringList& args,
                  const QMap<QString, QString>& extraEnv,
-                 int timeoutMs = -1);
+                 int timeoutMs = -1,
+                 QByteArray* captured = nullptr);
 
   /// Run a plain host command (NO SLR wrapping).
   /// Used for host utilities like curl, unzip that must not run in the container.