initial draft, adding custom page after welcome page if the absence of required deps is detected, installation process will abort either way, but the user gets to choose if they want to download the deps now or later

installer/win/ghost-trap.iss

@@ -1,4 +1,4 @@
-; Copyright (c) 2012-2023 PaperCut Software Pty Ltd
+; Copyright (c) 2012-2024 PaperCut Software Pty Ltd
 ; Author: Chris Dance <chris.dance@papercut.com>
 ; 
 ; License: GNU Affero GPL v3 - See project LICENSE file.
@@ -84,6 +84,22 @@ Filename: {app}\bin\{#gs_c_exe}; Parameters: "-q -dBATCH ""-sFONTDIR={code:Fonts
 Type: filesandordirs; Name: {app}\lib\cidfmap;
 
 [Code]
+
+const
+; Expected minimum version is 14.38.33130.00, for Visual C++ 2015 redistributable packs
+; Update the values here if our requirements change over time
+  MinMajor = 14;
+  { only applies if actual major version is 14, do not apply if actual major version is higher }
+  MinMinor = 38;
+  { only applies if actual major is 14 and actual minor 38, do not apply if previous versions are higher }
+  MinBld = 33130;
+
+; Used to store value of whether we have acceptable Visual C++ Runtime components
+var
+  HasAcceptableVCRuntime : Boolean;
+  VCRuntimeMissingOptionsPage : TInputOptionWizardPage;
+  IsInstallationFinishedNormally : Boolean;
+
 function FontsDirWithForwardSlashes(Param: String): String;
 begin
   { The system's Fonts directory in forward slash format }
@@ -116,3 +132,88 @@ begin
     Exec(ExpandConstant(sUnInstallString), '/VERYSILENT', '', SW_SHOW, ewWaitUntilTerminated, iResultCode);
   end;
 end;
+
+function IsAcceptableVCRuntimeInstalled() : Boolean;
+var
+  cMajor: Cardinal;
+  cMinor: Cardinal;
+  cBld: Cardinal;
+  cRbld: Cardinal;
+  sKey: String;
+begin
+  Result := False;
+  ; for more info see https://learn.microsoft.com/en-us/cpp/windows/redistributing-visual-cpp-files?view=msvc-170#install-the-redistributable-packages
+  sKey := 'SOFTWARE\Microsoft\VisualStudio\14.0\VC\Runtimes\x64';
+  if RegQueryDWordValue(HKEY_LOCAL_MACHINE, sKey, 'Major', cMajor) then begin
+    if RegQueryDWordValue(HKEY_LOCAL_MACHINE, sKey, 'Minor', cMinor) then begin
+      if RegQueryDWordValue(HKEY_LOCAL_MACHINE, sKey, 'Bld', cBld) then begin
+        if RegQueryDWordValue(HKEY_LOCAL_MACHINE, sKey, 'RBld', cRbld) then begin
+          if cMajor > MinMajor then begin
+            Result := True;
+            Exit;
+          end;
+
+          if cMajor < MinMajor then begin
+            Exit;
+          end;
+
+          if cMinor > MinMinor then begin
+            Result := True;
+            Exit;
+          end;
+
+          if cMinor < MinMinor then begin
+            Exit;
+          end;
+
+          if (cBld >= MinBld) and (cRbld >= 0) then begin
+            Result := True;
+          end;
+        end;
+      end;
+    end;
+  end;
+end;
+
+procedure InitializeWizard();
+begin
+  HasAcceptableVCRuntime := IsAcceptableVCRuntimeInstalled();
+; If the OS is missing required dependencies, the installation process will not go the full length and will be aborted early
+  if not HasAcceptableVCRuntime then begin
+    IsInstallationFinishedNormally := False;
+    VCRuntimeMissingOptionsPage := CreateInputOptionPage(wpWelcome,
+      'Visual C++ Runtime Required',
+      'You are seeing this page because required dependencies for GhostTrap are missing on your operating system.',
+      'GhostTrap of version 1.4.10.02 and above requires reasonably up-to-date Visual C++ Runtimes to function properly. You must install required Microsoft Redistributables before installing GhostTrap.' + #13#10 + #13#10 + 'Either option will terminate the current installation process.' + #13#10 + 'Read more about this: https://learn.microsoft.com/en-us/cpp/windows/latest-supported-vc-redist?view=msvc-170#visual-studio-2015-2017-2019-and-2022',
+      False, False);
+
+    VCRuntimeMissingOptionsPage.Add('Install the dependencies from Microsoft now. (Recommended)');
+    VCRuntimeMissingOptionsPage.Add('I will find the required dependencies myself later.');
+  end;
+; If the OS has the required dependencies, completion of the installation is treated as normal
+  else
+  begin
+    IsInstallationFinishedNormally := True;
+  end;
+end;
+
+procedure CurStepChanged(CurStep : TSetupStep);
+begin
+  if curStep = ssPostInstall then begin
+    if not WizardForm.Canceled then begin
+      if not IsInstallationFinishedNormally then begin
+        case VCRuntimeMissingOptionsPage.Values[0] of
+        0: begin
+          { install dependencies now, using Microsoft's perma-link }
+          ShellExec('', 'https://aka.ms/vs/17/release/vc_redist.x64.exe');
+          Abort;
+        end;
+        1: begin
+          { do it later themselves }
+          MsgBox('Please install required VC++ dependencies first and try again.', mbInformation, MB_OK);
+          Abort;
+        end;
+      end;
+    end;
+  end;
+end;