[FHIR Connector] Add a general proxy function to forward requests to FHIR server with url rewrite

fhir/fhir_connector.bal

@@ -983,6 +983,31 @@ public isolated client class FHIRConnector {
             return error(string `${FHIR_CONNECTOR_ERROR}: ${e.message()}`, errorDetails = e);
         }
     }
+
+    # Proxies an incoming HTTP request to the FHIR server and returns the upstream response as-is.
+    #
+    # + requestUrl - Relative request URL to be forwarded to the configured FHIR server
+    # + request - Incoming HTTP request to be forwarded as-is
+    # + return - The upstream HTTP response, with URL rewriting applied when enabled.
+    @display {label: "Proxy request to FHIR server"}
+    remote isolated function proxy(@display {label: "Request URL"} string requestUrl,
+            @display {label: "Incoming Request"} http:Request request)
+                                    returns http:Response|FHIRError {
+        do {
+            log:printDebug(string `Request URL: ${requestUrl}`);
+            http:Response result = check self.httpClient->forward(requestUrl, check enrichRequest(request, self.pkjwtHanlder));
+            if self.urlRewrite {
+                return generalRewriteServerUrl(result, self.baseUrl, self.replacementURL);
+            }
+            return result;
+        } on fail error e {
+            log:printDebug(string `${FHIR_CONNECTOR_ERROR}: ${e.message()}`, e);
+            if e is FHIRError {
+                return e;
+            }
+            return error(string `${FHIR_CONNECTOR_ERROR}: ${e.message()}`, errorDetails = e);
+        }
+    }
 }
 
 # Waits for the completion of a bulk export file download.

fhir/utils.bal

@@ -206,7 +206,7 @@ isolated function getBundleResponse(http:Response response) returns FHIRResponse
         int statusCode = response.statusCode;
         json|xml responseBody = check extractResponseBody(response);
         map<string> responseHeaders = extractHeadersFromResponse(response);
-            if (is2xx(statusCode)) {
+        if (is2xx(statusCode)) {
             FHIRResponse fhirResponse = {httpStatusCode: statusCode, 'resource: responseBody, serverResponseHeaders: responseHeaders};
             return fhirResponse;
         } else {
@@ -598,6 +598,69 @@ isolated function rewriteServerUrl(FHIRResponse response, string baseUrl, string
     }
 }
 
+// Replaces base url in all headers and payload content of a generic http:Response.
+isolated function generalRewriteServerUrl(http:Response response, string baseUrl, string? replacementUrl = ()) returns http:Response|FHIRError {
+    if replacementUrl is () {
+        return response;
+    }
+
+    do {
+        var rewrite = isolated function(string inputString) returns string {
+            return re `${baseUrl}`.replaceAll(inputString, <string>replacementUrl);
+        };
+
+        // Rewrite all header values (including multi-value headers)
+        foreach string headerName in response.getHeaderNames() {
+            string[] values = check response.getHeaders(headerName);
+            if values.length() > 0 {
+                response.removeHeader(headerName);
+                foreach string value in values {
+                    response.addHeader(headerName, rewrite(value));
+                }
+            }
+        }
+
+        string contentType = "";
+        string contentTypeCheck = "";
+        string|error contentTypeResult = response.getHeader(http:CONTENT_TYPE);
+        if contentTypeResult is string {
+            contentType = contentTypeResult;
+            contentTypeCheck = contentType.toLowerAscii();
+        }
+
+        // Rewrite payload based on type; fallback to text when possible.
+        if contentTypeCheck.includes("json") {
+            json payload = check response.getJsonPayload();
+            string rewritten = rewrite(payload.toJsonString());
+            json rewrittenJson = check rewritten.fromJsonString();
+            response.setPayload(rewrittenJson);
+            return response;
+        }
+
+        if contentTypeCheck.includes("xml") {
+            xml payload = check response.getXmlPayload();
+            string rewritten = rewrite(payload.toString());
+            xml rewrittenXml = check xml:fromString(rewritten);
+            response.setPayload(rewrittenXml);
+            return response;
+        }
+
+        string|error textPayload = response.getTextPayload();
+        if textPayload is string {
+            string rewrittenText = rewrite(textPayload);
+            if contentType == "" {
+                response.setTextPayload(rewrittenText);
+            } else {
+                response.setTextPayload(rewrittenText, contentType = contentType);
+            }
+        }
+
+        return response;
+    } on fail var e {
+        return error FHIRConnectorError(string `${FHIR_CONNECTOR_ERROR}: ${e.message()}`, errorDetails = e);
+    }
+}
+
 isolated function constructHttpConfigs(FHIRConnectorConfig|BulkExportConfig config) returns http:ClientConfiguration {
     http:ClientConfiguration httpConfig = {
         httpVersion: config.httpVersion,