re-register OpenWithProgids at startup if installed

This restores SumatraPDF in the 'Open with' context menu for .pdf (and other
supported formats) in case another app (e.g. Microsoft Edge) has removed the
entry after taking over the default handler.

Author: kjk

src/Installer.h

@@ -92,3 +92,5 @@ bool WriteExtendedFileExtensionInfo(HKEY hkey, const char* installedExePat);
 bool RemoveUninstallerRegistryInfo(HKEY hkey);
 void RemoveInstallRegistryKeys(HKEY hkey);
 int GetInstallerWinDx();
+
+void ReRegisterFileAssociations();

src/RegistryInstaller.cpp

@@ -32,6 +32,22 @@ static void ShellNotifyAssociationsChanged() {
     SHChangeNotify(SHCNE_ASSOCCHANGED, SHCNF_IDLIST, nullptr, nullptr);
 }
 
+static bool HasRegistryValue(HKEY hkey, const char* keyName, const char* valName) {
+    WCHAR* keyW = ToWStrTemp(keyName);
+    WCHAR* valW = ToWStrTemp(valName);
+    DWORD type = 0;
+    DWORD cb = 0;
+    LSTATUS res = SHGetValueW(hkey, keyW, valW, &type, nullptr, &cb);
+    // success or ERROR_MORE_DATA means the value exists
+    return (res == ERROR_SUCCESS || res == ERROR_MORE_DATA);
+}
+
+static bool HasOurOpenWithEntry(HKEY hkey, const char* ext) {
+    TempStr key = str::JoinTemp("Software\\Classes\\", ext, "\\OpenWithProgids");
+    TempStr progID = str::JoinTemp(kAppName, ext);
+    return HasRegistryValue(hkey, key, progID);
+}
+
 // caller needs to str::Free() the result
 static char* GetInstallDate() {
     SYSTEMTIME st;
@@ -635,3 +651,44 @@ void RemoveInstallRegistryKeys(HKEY hkey) {
 
     ShellNotifyAssociationsChanged();
 }
+
+// re-register our "Open With" file association handlers (under OpenWithProgids
+// and the corresponding ProgID entries) if this is an installed (non-portable)
+// copy of SumatraPDF. We do this at startup to counter other apps (e.g. Microsoft
+// Edge) that might remove us from the "Open with" context menu for .pdf etc. files.
+// We only touch HKCU (always writable by the current user) and optionally HKLM
+// (for all-users installs; fails gracefully without admin rights).
+void ReRegisterFileAssociations() {
+    if (!IsOurExeInstalled()) {
+        return;
+    }
+    TempStr exePath = GetSelfExePathTemp();
+    if (str::IsEmpty(exePath)) {
+        return;
+    }
+
+    bool didRegister = false;
+    if (!HasOurOpenWithEntry(HKEY_CURRENT_USER, ".pdf")) {
+        RegisterForOpenWith(HKEY_CURRENT_USER, exePath);
+        if (IsWindows10OrGreater()) {
+            RegisterForDefaultPrograms(HKEY_CURRENT_USER, exePath);
+        }
+        didRegister = true;
+    }
+
+    // for all-users installs, also try to restore the HKLM entries (best effort)
+    TempStr regPathUninst = GetRegPathUninstTemp(kAppName);
+    if (HasRegistryValue(HKEY_LOCAL_MACHINE, regPathUninst, "InstallLocation")) {
+        if (!HasOurOpenWithEntry(HKEY_LOCAL_MACHINE, ".pdf")) {
+            RegisterForOpenWith(HKEY_LOCAL_MACHINE, exePath);
+            if (IsWindows10OrGreater()) {
+                RegisterForDefaultPrograms(HKEY_LOCAL_MACHINE, exePath);
+            }
+            didRegister = true;
+        }
+    }
+
+    if (didRegister) {
+        ShellNotifyAssociationsChanged();
+    }
+}

src/SumatraStartup.cpp

@@ -1510,6 +1510,7 @@ int APIENTRY WinMain(_In_ HINSTANCE /*hInstance*/, _In_opt_ HINSTANCE, _In_ LPST
     }
 
     DetectExternalViewers();
+    ReRegisterFileAssociations();
 
     gRenderCache = new RenderCache();