WIP: Add support for side-by-side PS Core preview on Linux/macOS (#1366)

* Add support for side-by-side PS Core preview on Linux/macOS

This PR supports listing the symlinks for pwsh & pwsh-preview.

The preferred default version is pwsh if both are present however, the
default will be pwsh-preview if it's installed and pwsh isn't.

So this only support side-by-side on Linux/macOS up to two versions:
stable and preview.

* Adjust name of default PS Core on Linux/macOS

* Make a bit DRY-er. Thx @tylerl0706

* Remove support for pre-ship PS Core Linux/macOS binary name - powershell

* Add support for PowerShell Core installed in a Snap

* Simplify impl of getting available PS on Linux/macOS

src/platform.ts

@@ -3,12 +3,19 @@
  *--------------------------------------------------------*/
 
 import fs = require("fs");
-import os = require("os");
 import path = require("path");
 import process = require("process");
-import vscode = require("vscode");
 import Settings = require("./settings");
 
+const linuxExePath        = "/usr/bin/pwsh";
+const linuxPreviewExePath = "/usr/bin/pwsh-preview";
+
+const snapExePath         = "/snap/bin/pwsh";
+const snapPreviewExePath  = "/snap/bin/pwsh-preview";
+
+const macOSExePath        = "/usr/local/bin/pwsh";
+const macOSPreviewExePath = "/usr/local/bin/pwsh-preview";
+
 export enum OperatingSystem {
     Unknown,
     Windows,
@@ -47,6 +54,13 @@ export function getPlatformDetails(): IPlatformDetails {
     };
 }
 
+/**
+ * Gets the default instance of PowerShell for the specified platform.
+ * On Windows, the default version of PowerShell is "Windows PowerShell".
+ * @param platformDetails Specifies information about the platform - primarily the operating system.
+ * @param use32Bit On Windows, this boolean determines whether the 32-bit version of Windows PowerShell is returned.
+ * @returns A string containing the path of the default version of PowerShell.
+ */
 export function getDefaultPowerShellPath(
     platformDetails: IPlatformDetails,
     use32Bit: boolean = false): string | null {
@@ -68,14 +82,21 @@ export function getDefaultPowerShellPath(
                     : SysnativePowerShellPath;
         }
     } else if (platformDetails.operatingSystem === OperatingSystem.MacOS) {
-        powerShellExePath = "/usr/local/bin/powershell";
-        if (fs.existsSync("/usr/local/bin/pwsh")) {
-            powerShellExePath = "/usr/local/bin/pwsh";
+        // Always default to the stable version of PowerShell (if installed) but handle case of only Preview installed
+        powerShellExePath = macOSExePath;
+        if (!fs.existsSync(macOSExePath) && fs.existsSync(macOSPreviewExePath)) {
+            powerShellExePath = macOSPreviewExePath;
         }
     } else if (platformDetails.operatingSystem === OperatingSystem.Linux) {
-        powerShellExePath = "/usr/bin/powershell";
-        if (fs.existsSync("/usr/bin/pwsh")) {
-            powerShellExePath = "/usr/bin/pwsh";
+        // Always default to the stable version of PowerShell (if installed) but handle case of only Preview installed
+        // as well as the Snaps case - https://snapcraft.io/
+        powerShellExePath = linuxExePath;
+        if (!fs.existsSync(linuxExePath) && fs.existsSync(linuxPreviewExePath)) {
+            powerShellExePath = linuxPreviewExePath;
+        } else if (fs.existsSync(snapExePath)) {
+            powerShellExePath = snapExePath;
+        } else if (fs.existsSync(snapPreviewExePath)) {
+            powerShellExePath = snapPreviewExePath;
         }
     }
 
@@ -108,6 +129,12 @@ export function fixWindowsPowerShellPath(powerShellExePath: string, platformDeta
     return powerShellExePath;
 }
 
+/**
+ * Gets a list of all available PowerShell instance on the specified platform.
+ * @param platformDetails Specifies information about the platform - primarily the operating system.
+ * @param sessionSettings Specifies the user/workspace settings. Additional PowerShell exe paths loaded from settings.
+ * @returns An array of IPowerShellExeDetails objects with the PowerShell name & exe path for each instance found.
+ */
 export function getAvailablePowerShellExes(
     platformDetails: IPlatformDetails,
     sessionSettings: Settings.ISettings | undefined): IPowerShellExeDetails[] {
@@ -162,13 +189,25 @@ export function getAvailablePowerShellExes(
         }
     } else {
         // Handle Linux and macOS case
-        paths.push({
-            versionName: "PowerShell Core",
-            exePath: this.getDefaultPowerShellPath(platformDetails),
+        let exePaths: string[];
+
+        if (platformDetails.operatingSystem === OperatingSystem.Linux) {
+            exePaths = [ linuxExePath, snapExePath, linuxPreviewExePath, snapPreviewExePath ];
+        } else {
+            exePaths = [ macOSExePath, macOSPreviewExePath ];
+        }
+
+        exePaths.forEach((exePath) => {
+            if (fs.existsSync(exePath)) {
+                paths.push({
+                    versionName: "PowerShell Core" + (/-preview/.test(exePath) ? " Preview" : ""),
+                    exePath,
+                });
+            }
         });
     }
 
-    // When unit testing, we don't have session settings to test so skip reading this setting
+    // When unit testing, we don't have session settings available to test, so skip reading this setting
     if (sessionSettings) {
         // Add additional PowerShell paths as configured in settings
         for (const additionalPowerShellExePath of sessionSettings.powerShellAdditionalExePaths) {