browserutils: support $BROWSER variable on Linux

On Linux we now attempt to use the $BROWSER environment variable to
launch a URL in 'the default' browser. This is preferred ahead of any
'shell execute' handlers.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

src/shared/Core/BrowserUtils.cs

@@ -29,7 +29,19 @@ public static void OpenDefaultBrowser(IEnvironment environment, Uri uri)
             if (PlatformUtils.IsLinux())
             {
                 //
-                // On Linux, 'shell execute' utilities like xdg-open launch a process without
+                // On Linux we try and use the $BROWSER environment variable first if set.
+                // Otherwise, we try a set of known 'shell execute' utilities.
+                //
+                if (TryGetBrowserEnvironment(environment, out string browser))
+                {
+                    psi = new ProcessStartInfo(browser, url)
+                    {
+                        RedirectStandardOutput = true,
+                        RedirectStandardError = true,
+                    };
+                }
+                //
+                // Sadly, 'shell execute' utilities like xdg-open launch a process without
                 // detaching from the standard in/out descriptors. Some applications (like
                 // Chromium) write messages to stdout, which is currently hooked up and being
                 // consumed by Git, and cause errors.
@@ -41,17 +53,19 @@ public static void OpenDefaultBrowser(IEnvironment environment, Uri uri)
                 // We try and use the same 'shell execute' utilities as the Framework does,
                 // searching for them in the same order until we find one.
                 //
-                if (!TryGetLinuxShellExecuteHandler(environment, out string shellExecPath))
+                else if (!TryGetLinuxShellExecuteHandler(environment, out string shellExecPath))
                 {
                     throw new Exception("Failed to locate a utility to launch the default web browser.");
                 }
-
-                psi = new ProcessStartInfo(shellExecPath, url)
+                else
                 {
-                    RedirectStandardOutput = true,
-                    // Ok to redirect stderr for non-git-related processes
-                    RedirectStandardError = true
-                };
+                    psi = new ProcessStartInfo(shellExecPath, url)
+                    {
+                        RedirectStandardOutput = true,
+                        // Ok to redirect stderr for non-git-related processes
+                        RedirectStandardError = true
+                    };
+                }
             }
             else
             {
@@ -67,6 +81,22 @@ public static void OpenDefaultBrowser(IEnvironment environment, Uri uri)
             Process.Start(psi);
         }
 
+        public static bool TryGetBrowserEnvironment(IEnvironment env, out string value)
+        {
+            // We can check the BROWSER environment variable to see if a specific
+            // browser has been requested by the user.
+            if (PlatformUtils.IsLinux() || PlatformUtils.IsMacOS())
+            {
+                if (env.Variables.TryGetValue("BROWSER", out value) && !string.IsNullOrWhiteSpace(value))
+                {
+                    return true;
+                }
+            }
+
+            value = null;
+            return false;
+        }
+
         public static bool TryGetLinuxShellExecuteHandler(IEnvironment env, out string shellExecPath)
         {
             // One additional 'shell execute' utility we also attempt to use over the Framework