qt: Use temporary .part file names when creating archives

* lang/qt/src/util.h, lang/qt/src/util.cpp (class PartialFileGuard):
New.
* lang/qt/src/util.cpp (getRandomCharacters, createPartFileName): New.
* lang/qt/src/qgpgmeencryptarchivejob.cpp (encrypt_to_filename): Use
PartialFileGuard.
* lang/qt/src/qgpgmesignarchivejob.cpp (sign_to_filename): Ditto.
* lang/qt/src/qgpgmesignencryptarchivejob.cpp
(sign_encrypt_to_filename): Ditto.
--

When creating signed and/or encrypted archives, gpgtar now writes the
result to a temporary file name. On success, the archive is renamed to
the final file name. Otherwise, the (partially written) temporary file
is removed (if possible).

GnuPG-bug-id: 6721

lang/qt/src/qgpgmeencryptarchivejob.cpp

@@ -133,18 +133,23 @@ static QGpgMEEncryptArchiveJob::result_type encrypt_to_filename(Context *ctx,
                                                                 Context::EncryptionFlags flags,
                                                                 const QString &baseDirectory)
 {
+    PartialFileGuard partFileGuard{outputFileName};
+    if (partFileGuard.tempFileName().isEmpty()) {
+        return std::make_tuple(EncryptionResult{Error::fromCode(GPG_ERR_EEXIST)}, QString{}, Error{});
+    }
+
     Data outdata;
 #ifdef Q_OS_WIN
-    outdata.setFileName(outputFileName.toUtf8().constData());
+    outdata.setFileName(partFileGuard.tempFileName().toUtf8().constData());
 #else
-    outdata.setFileName(QFile::encodeName(outputFileName).constData());
+    outdata.setFileName(QFile::encodeName(partFileGuard.tempFileName()).constData());
 #endif
 
     const auto result = encrypt(ctx, recipients, paths, outdata, flags, baseDirectory);
     const auto &encryptionResult = std::get<0>(result);
-    if (encryptionResult.error().code()) {
-        // ensure that the output file is removed if the operation was canceled or failed
-        removeFile(outputFileName);
+    if (!encryptionResult.error().code()) {
+        // the operation succeeded -> save the result under the requested file name
+        partFileGuard.commit();
     }
 
     return result;

lang/qt/src/qgpgmesignarchivejob.cpp

@@ -138,18 +138,23 @@ static QGpgMESignArchiveJob::result_type sign_to_filename(Context *ctx,
                                                           const QString &outputFileName,
                                                           const QString &baseDirectory)
 {
+    PartialFileGuard partFileGuard{outputFileName};
+    if (partFileGuard.tempFileName().isEmpty()) {
+        return std::make_tuple(SigningResult{Error::fromCode(GPG_ERR_EEXIST)}, QString{}, Error{});
+    }
+
     Data outdata;
 #ifdef Q_OS_WIN
-    outdata.setFileName(outputFileName.toUtf8().constData());
+    outdata.setFileName(partFileGuard.tempFileName().toUtf8().constData());
 #else
-    outdata.setFileName(QFile::encodeName(outputFileName).constData());
+    outdata.setFileName(QFile::encodeName(partFileGuard.tempFileName()).constData());
 #endif
 
     const auto result = sign(ctx, signers, paths, outdata, baseDirectory);
     const auto &signingResult = std::get<0>(result);
-    if (signingResult.error().code()) {
-        // ensure that the output file is removed if the operation was canceled or failed
-        removeFile(outputFileName);
+    if (!signingResult.error().code()) {
+        // the operation succeeded -> save the result under the requested file name
+        partFileGuard.commit();
     }
 
     return result;

lang/qt/src/qgpgmesignencryptarchivejob.cpp

@@ -147,19 +147,27 @@ static QGpgMESignEncryptArchiveJob::result_type sign_encrypt_to_filename(Context
                                                                          Context::EncryptionFlags encryptionFlags,
                                                                          const QString &baseDirectory)
 {
+    PartialFileGuard partFileGuard{outputFileName};
+    if (partFileGuard.tempFileName().isEmpty()) {
+        return std::make_tuple(SigningResult{Error::fromCode(GPG_ERR_EEXIST)},
+                               EncryptionResult{Error::fromCode(GPG_ERR_EEXIST)},
+                               QString{},
+                               Error{});
+    }
+
     Data outdata;
 #ifdef Q_OS_WIN
-    outdata.setFileName(outputFileName.toUtf8().constData());
+    outdata.setFileName(partFileGuard.tempFileName().toUtf8().constData());
 #else
-    outdata.setFileName(QFile::encodeName(outputFileName).constData());
+    outdata.setFileName(QFile::encodeName(partFileGuard.tempFileName()).constData());
 #endif
 
     const auto result = sign_encrypt(ctx, signers, recipients, paths, outdata, encryptionFlags, baseDirectory);
     const auto &signingResult = std::get<0>(result);
     const auto &encryptionResult = std::get<1>(result);
-    if (signingResult.error().code() || encryptionResult.error().code()) {
-        // ensure that the output file is removed if the operation was canceled or failed
-        removeFile(outputFileName);
+    if (!signingResult.error().code() && !encryptionResult.error().code()) {
+        // the operation succeeded -> save the result under the requested file name
+        partFileGuard.commit();
     }
 
     return result;

lang/qt/src/util.cpp

@@ -40,6 +40,8 @@
 #include "qgpgme_debug.h"
 
 #include <QFile>
+#include <QFileInfo>
+#include <QRandomGenerator>
 
 #include <key.h>
 
@@ -76,3 +78,106 @@ void removeFile(const QString &fileName)
         }
     }
 }
+
+/**
+ * Generates a string of random characters for the file names of temporary files.
+ * Never use this for generating passwords or similar use cases requiring highly
+ * secure random data.
+ */
+static QString getRandomCharacters(const int count)
+{
+    if (count < 0) {
+        return {};
+    }
+
+    QString randomChars;
+    randomChars.reserve(count);
+
+    do {
+        // get a 32-bit random number to generate up to 5 random characters from
+        // the set {A-Z, a-z, 0-9}; set the highest bit for the break condition
+        for (quint32 rnd = QRandomGenerator::global()->generate() | (1 << 31); rnd > 3; rnd = rnd >> 6)
+        {
+            // take the last 6 bits; ignore 62 and 63
+            const char ch = rnd & ((1 << 6) - 1);
+            if (ch < 26) {
+                randomChars += QLatin1Char(ch + 'A');
+            } else if (ch < 26 + 26) {
+                randomChars += QLatin1Char(ch - 26 + 'a');
+            } else if (ch < 26 + 26 + 10) {
+                randomChars += QLatin1Char(ch - 26 - 26 + '0');
+            }
+            if (randomChars.size() >= count) {
+                break;
+            }
+        }
+    } while (randomChars.size() < count);
+
+    return randomChars;
+}
+
+/**
+ * Creates a temporary file name with extension \c .part for the given file name
+ * \a fileName. The function makes sure that the created file name is not in use
+ * at the time the file name is chosen.
+ *
+ * Example: For the file name "this.is.an.archive.tar.gpg" the temporary file name
+ * "this.YHgf2tEl.is.an.archive.tar.gpg.part" could be returned.
+ */
+static QString createPartFileName(const QString &fileName)
+{
+    static const int maxAttempts = 10;
+
+    const QFileInfo fi{fileName};
+    const QString path = fi.path(); // path without trailing '/'
+    const QString baseName = fi.baseName();
+    const QString suffix = fi.completeSuffix();
+    for (int attempt = 0; attempt < maxAttempts; ++attempt) {
+        const QString candidate = (path + QLatin1Char('/')
+                                   + baseName + QLatin1Char('.')
+                                   + getRandomCharacters(8) + QLatin1Char('.')
+                                   + suffix
+                                   + QLatin1String(".part"));
+        if (!QFile::exists(candidate)) {
+            return candidate;
+        }
+    }
+
+    qCWarning(QGPGME_LOG) << __func__ << "- Failed to create temporary file name for" << fileName;
+    return {};
+}
+
+PartialFileGuard::PartialFileGuard(const QString &fileName)
+    : mFileName{fileName}
+    , mTempFileName{createPartFileName(fileName)}
+{
+    qCDebug(QGPGME_LOG) << __func__ << "- Using temporary file name" << mTempFileName;
+}
+
+PartialFileGuard::~PartialFileGuard()
+{
+    if (!mTempFileName.isEmpty()) {
+        removeFile(mTempFileName);
+    }
+}
+
+QString PartialFileGuard::tempFileName() const
+{
+    return mTempFileName;
+}
+
+bool PartialFileGuard::commit()
+{
+    if (mTempFileName.isEmpty()) {
+        qCWarning(QGPGME_LOG) << "PartialFileGuard::commit: Called more than once";
+        return false;
+    }
+    const bool success = QFile::rename(mTempFileName, mFileName);
+    if (success) {
+        qCDebug(QGPGME_LOG) << __func__ << "- Renamed" << mTempFileName << "to" << mFileName;
+        mTempFileName.clear();
+    } else {
+        qCDebug(QGPGME_LOG) << __func__ << "- Renaming" << mTempFileName << "to" << mFileName << "failed";
+    }
+    return success;
+}

lang/qt/src/util.h

@@ -57,4 +57,26 @@ QStringList toFingerprints(const std::vector<GpgME::Key> &keys);
 
 void removeFile(const QString &fileName);
 
+/**
+ * Helper for using a temporary "part" file for writing a result to, similar
+ * to what browsers do when downloading files.
+ * On success, you commit() which renames the temporary file to the
+ * final file name. Otherwise, you do nothing and let the helper remove the
+ * temporary file on destruction.
+ */
+class PartialFileGuard
+{
+public:
+    explicit PartialFileGuard(const QString &fileName);
+    ~PartialFileGuard();
+
+    QString tempFileName() const;
+
+    bool commit();
+
+private:
+    QString mFileName;
+    QString mTempFileName;
+};
+
 #endif // __QGPGME_UTIL_H__