add a very limited INCLUDE_ONLY option

Dockerfile

@@ -0,0 +1,41 @@
+#FROM debian:bookworm-20231030-slim
+FROM debian:bullseye-20220801
+
+# need to add
+      # removed
+      # libsasl2-modules \
+      # ca-certificates \
+
+# version pinning is being handled in our from line
+# hadolint ignore=DL3008
+RUN true && \
+    apt-get update && \
+    groupadd --gid 1000 user && \
+    useradd -m --home-dir /home/user --shell /bin/sh --uid 1000 --gid 1000 user && \
+    apt-get install -y --no-install-recommends \
+      libsasl2-2 \
+      libsasl2-dev \
+      perl \
+      libdatetime-format-dateparse-perl \
+      autoconf \
+      automake \
+      zlib1g-dev \
+      libdb-dev \
+      libsasl2-dev \
+      libssl-dev \
+      gcc \
+      make \
+      git \
+      && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/* && \
+    true
+
+# Built with
+# apt install build-essential dh-autoreconf git
+# apt install libssl-dev
+# apt install zlib1g-dev
+# apt install libsasl2-dev
+
+WORKDIR /home/user
+USER user

build

@@ -0,0 +1,3 @@
+#!/bin/sh
+# Note we assume this is run with podman. Take -u 0:0 out if using docker
+docker run -u 0:0 -it --rm -v $(pwd):/make -w /make isync-build make 

src/drv_maildir.c

@@ -920,6 +920,28 @@ maildir_compare( const void *l, const void *r )
 	return strcmp( lm->base, rm->base );
 }
 
+int is_gmail(const char* name) {
+	// NOTE: This is not an exact science. For instance, there is a good
+	// shot that mail sent from containers will end up matching this pattern
+    char token[16];  // If host name is longer than 12, then we already have our answer
+
+	// we expect files in the form of: "1700172103.R12128272304961211247.hostname,U=27:2,S"
+	// we are looking for "hostname"
+	// All gmail mails have a hostname in the form of "ff6d55f971cc".
+	// All are 12 characters long
+	// All are hexadecimal
+    if (sscanf(name, "%*[^.].%*[^.].%15[^,]", token) == 1) {
+		token[14] = '\0';
+		size_t len = strlen(token);
+		if (len != 12) return 0;
+		if (strspn(token, "0123456789abcdef") == len) return 1;
+    }
+
+	// We are here because a) the input format is not as expected,
+	// or b) the hostname is not gmail
+    return 0;
+}
+
 static int
 maildir_scan( maildir_store_t *ctx, msg_t_array_alloc_t *msglist )
 {
@@ -992,11 +1014,24 @@ maildir_scan( maildir_store_t *ctx, msg_t_array_alloc_t *msglist )
 				return DRV_BOX_BAD;
 			}
 			const char *filter = getenv("MBSYNC_MAILDIR_IGNORE");
+			const char *MAGIC_INCLUDE = "^[^\\.]+\\.[^\\.]+\\.[0-9a-f]{12}\\,.*";
+			char *include = getenv("MBSYNC_MAILDIR_INCLUDE_ONLY");
+			if (include && strncmp(MAGIC_INCLUDE, include, strlen(MAGIC_INCLUDE)) != 0) {
+				include = NULL;
+				error("MBSYNC_MAILDIR_INCLUDE_ONLY can only be '%s'\n", MAGIC_INCLUDE);
+			}
+			if (include && filter) {
+				include = NULL;
+				error("MBSYNC_MAILDIR_IGNORE cannot be used with MBSYNC_MAILDIR_INCLUDE_ONLY. INCLUDE_ONLY will be ignored\n");
+			}
+
 			while ((e = readdir( d ))) {
 				if (*e->d_name == '.')
 					continue;
 				if (filter && strstr(e->d_name, filter))
 					continue;
+				if (include && !is_gmail(e->d_name))
+					continue;
 				ctx->total_msgs++;
 				ctx->recent_msgs += i;
 #ifdef USE_DB