[GR-74994] Restore JDK 21 compatibility in CertUtils

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/ssl/CertUtils.java

@@ -733,7 +733,7 @@ static PrivateKey getPrivateKey(PythonContext context, Node inliningTarget, PCon
         PrivateKey privateKey = null;
         String algorithm = cert.getPublicKey().getAlgorithm();
         try {
-            String pemText = reader.readAllAsString();
+            String pemText = readAll(reader);
             int fromIndex = 0;
             PemBlockWithContent rawBlock;
             while ((rawBlock = findNextPemBlock(pemText, fromIndex)) != null) {
@@ -773,6 +773,16 @@ static PrivateKey getPrivateKey(PythonContext context, Node inliningTarget, PCon
         return privateKey;
     }
 
+    private static String readAll(BufferedReader reader) throws IOException {
+        StringBuilder sb = new StringBuilder();
+        char[] buf = new char[8192];
+        int n;
+        while ((n = reader.read(buf)) != -1) {
+            sb.append(buf, 0, n);
+        }
+        return sb.toString();
+    }
+
     private static PemBlockWithContent findNextPemBlock(String data, int fromIndex) throws IOException {
         int begin = data.indexOf("-----BEGIN ", fromIndex);
         if (begin < 0) {