Use assembly API to build executable path

src/powershell/Program.cs

@@ -114,13 +114,13 @@ private static bool HasLoginSpecified(string[] args, out int loginIndex)
         private static int ExecPwshLogin(string[] args, int loginArgIndex)
         {
             // We need the path to the current pwsh executable
-            string pwshPath = System.Diagnostics.Process.GetCurrentProcess().MainModule.FileName;
+            string pwshDllPath = Assembly.GetEntryAssembly().Location;
 
             // Create input for /bin/sh that execs pwsh
-            int quotedPwshPathLength = GetQuotedPathLength(pwshPath);
+            int quotedPwshPathLength = GetQuotedPwshPathLength(pwshDllPath);
             string pwshInvocation = string.Create(
                 10 + quotedPwshPathLength, // exec '{pwshPath}' "$@"
-                (pwshPath, quotedPwshPathLength),
+                (pwshDllPath, quotedPwshPathLength),
                 CreatePwshInvocation);
 
             // Set up the arguments for /bin/sh
@@ -162,24 +162,32 @@ private static int ExecPwshLogin(string[] args, int loginArgIndex)
         }
 
         /// <summary>
-        /// Gets what the length of the given string will be if it's
+        /// Gets what the length of the path to pwsh will be when it's
         /// quote escaped for /bin/sh.
         /// </summary>
-        /// <param name="str">The string to quote escape.</param>
+        /// <param name="pwshDllPath">The path to pwsh.dll.</param>
         /// <returns>The length of the string when it's quote escaped.</returns>
-        private static int GetQuotedPathLength(string str)
+        private static int GetQuotedPwshPathLength(string pwshDllPath)
         {
             int length = 2;
-            foreach (char c in str)
+            foreach (char c in pwshDllPath)
             {
                 length++;
                 if (c == '\'') { length++; }
             }
 
-            return length;
+            // Subtract 4, since that's the difference between
+            // "pwsh" and "pwsh.dll", of which we need the former
+            return length - 4;
         }
 
-        private static void CreatePwshInvocation(Span<char> strBuf, (string path, int quotedLength) pwshPath)
+        /// <summary>
+        /// Creates the invocation string for pwsh in the exec script.
+        /// This method is to be used as the SpanAction in a string.Create call.
+        /// </summary>
+        /// <param name="strBuf">The internal string buffer to write to.</param>
+        /// <param name="pwshDllPath">A value tuple containing the pwsh.dll path and the anticipated quoted length.</param>
+        private static void CreatePwshInvocation(Span<char> strBuf, (string path, int quotedLength) pwshDllPath)
         {
             // "exec "
             strBuf[0] = 'e';
@@ -189,11 +197,11 @@ private static void CreatePwshInvocation(Span<char> strBuf, (string path, int qu
             strBuf[4] = ' ';
 
             // The quoted path to pwsh, like "'/opt/microsoft/powershell/7/pwsh'"
-            Span<char> pathSpan = strBuf.Slice(5, pwshPath.quotedLength);
-            QuoteAndWriteToSpan(pwshPath.path, pathSpan);
+            Span<char> pathSpan = strBuf.Slice(5, pwshDllPath.quotedLength);
+            QuoteAndWritePwshPathToSpan(pwshDllPath.path, pathSpan);
 
             // ' "$@"' the argument vector splat to pass pwsh arguments through
-            int argIndex = 5 + pwshPath.quotedLength;
+            int argIndex = 5 + pwshDllPath.quotedLength;
             strBuf[argIndex]     = ' ';
             strBuf[argIndex + 1] = '"';
             strBuf[argIndex + 2] = '$';
@@ -202,19 +210,21 @@ private static void CreatePwshInvocation(Span<char> strBuf, (string path, int qu
         }
 
         /// <summary>
-        /// Quotes (and sh quote escapes) a string and writes it to the given span.
+        /// Writes the sh-quote-escaped path to pwsh into the given span.
         /// </summary>
-        /// <param name="arg">The string to quote.</param>
+        /// <param name="pwshDllPath">The path to pwsh.dll</param>
         /// <param name="span">The span to write to.</param>
-        private static void QuoteAndWriteToSpan(string arg, Span<char> span)
+        private static void QuoteAndWritePwshPathToSpan(string pwshDllPath, Span<char> span)
         {
+            int psHomeEndIndex = pwshDllPath.LastIndexOf('/');
+
             span[0] = '\'';
 
             int i = 0;
             int j = 1;
-            for (; i < arg.Length; i++, j++)
+            for (; i < psHomeEndIndex; i++, j++)
             {
-                char c = arg[i];
+                char c = pwshDllPath[i];
 
                 if (c == '\'')
                 {
@@ -226,7 +236,12 @@ private static void QuoteAndWriteToSpan(string arg, Span<char> span)
                 span[j] = c;
             }
 
-            span[j] = '\'';
+            span[j++] = '/';
+            span[j++] = 'p';
+            span[j++] = 'w';
+            span[j++] = 's';
+            span[j++] = 'h';
+            span[j]   = '\'';
         }
 
         /// <summary>